Quantus

That's fair, I love 3.5 too, I've just had to slowly get used to the idea that most people mean 5e these days. (Literal 3.5, or Pathfinder?, btw). The only thing I can really say for 5E is that the Advantage/Disadvantage system makes it simpler to adapt oddball things on the fly when you're trying adapt make a new setting, but that's all to DM preference. I have an overall suggestion for General Feruchemy: One for One Bonus to Penalty Per Roll, banked for later. So like any reasonable Feruchemist you will choose the best place and time to Store, and it can represent an extended period of Vulnerability where thematically appropriate, etc but mechanically you make a Roll with a penalty and that banks an equivalent number of Bonus points to be spent Tapping on a later Roll. So if you are storing [Blank], you choose how much to attempt to store (ie the numeric Penalty) and then you will always make some Roll with whatever situational modifiers the DM feels are appropriate for the measures the Players have come up with(lying in Bed, being actively monitored by a Nurse, Supplemental O2 or Heat, etc). Regardless of the outcome you will have banked those Feruchemical Points, but failing means some consequence: While storing lots of Strength you failed to walk across your own bedroom and fell, taking a bit or damage. Or while storing Health you failed the roll and got the Flu. Or while storing Breath you started to hyperventilate and instead of Storing Points you just had to restart your Storing session with a smaller Penalty. Or while storing Physical Speed laying in bed listening to the radio you'll Not Move even harder with no real consequence at all. Once you have the Stored Points, you can use as many as you want to add to any single Roll in the future (appropriate to the metal). This way they get to be creative on when and how they Store to mitigate the downside, but if they try to do it on they fly it'll integrate into the story and balance relatively naturally. You'll have to work out some standard exchange rate for things like Gold where t can be cashed out for Constitution/Fortitude Rolls but also for direct HP healing. And the exchange rates for Compounding will need to be looked a little closer if you plan to have a lot of hemalurgy or twinborn, but twinborn and Fullborn are supoosed to be OP.

It also might depend on where in the natural process the feruchemy plugs in. Training with pure O2 may have limited benefits because your blood can only absorb so much so fast, but directly increasing your Hemaglobin and/or red bloodcell counts through training or blood doping are proven to work for artificially increasing your body's ability to absorb and deliver O2 to your tissues. It's not uncommon to train in high altitudes or with a restrictive face-mask to naturally increase your Blood's ability to absorb O2, so continuously storing would probably have that long-term benefit. If it goes full anime with the fantasy powers: Literal Fire Breath from being able to exhale nearly 100% O2 with an unavoidable dash aerosolized organic matter for fuel (takes surprisingly little near pure O2). On General F-Cadmium usefulness, I have no proof at all but I'd like to think it's possible to store any Gaseous Investiture in the metalmind, including Stormlight or Breaths, making it a great (non-leaky & metal) storage. And it will almost certainly be a key part of the Scadrian space fleet's life support systems.

As far as what it would look like from a Realmic and/or Investiture sense, it's hard to tell. Endowment literally just carves a massive chunk of Shardic Investiture and makes them, and the nature of Endowment makes it a pretty clean "No Strings Attached" state but requires regular fuel. Heralds are another example but that was a much more specific Pact with lots of different Conditions, Connections, and goals. It seems like they are sustained through the Oathpact itself but it's entirely possible that it's sustained by the ambient Investiture on Roshar and/or Braize or some such renewed Fuel. Preservation would probably just Connect them to the Shard permanently for the Power in a unique sort of Lerasium-esk effect. Ruin magic seems to prefer cannibalizing something existing instead of providing something new, so it could be anything from a Spike to staple Power directly, to a useful Blockage like what led to Kandra, or even a Vampire that sustains itself by sucking Feruchemical traits from sapient victims (instead of eating Breaths). I like that last one best for Harmony, personally.

(5e Dnd?) F-Zinc is not just inferior Speed (though there is some Bullet Time overlap per WOB), or even just Speed of Thought, it's an overall increase in Intelligence and both processing speed and power, including Intuitive Leaps (which is huge). So I'd say it should grant advantage on all Int-based checks, something akin to Bardic Knowledge, and all the effects of the Keen Mind feat (other than the Memory thing which would be covered by Copper). I wouldnt necessarily give them a Bonus to Initiative (that's more the physical speed and F-tin for the wisdom/Perception side) but I'd give them the "Cannot be Surprised" part of the Alert feat (they arent surprised but may not be capable of physically acting first). I also wouldnt give it an innate Attack or damage bonus, though if they can add Int to either from some other class ability I'd let the Advantage apply. For A-Gold, it's explicitly not a great metal so it's going to have some side effects. Up side is they can perceive through their Shadow's eyes and mind, so there would be a second token on the field with it's own line of sight. That 2nd person is a little like a ghost since only you can see or touch them, and also gives you some character changes along the lines of a DM designed Forgery to represent your current Possible Alternate Self. Combined with F-Zinc and you could really leverage that alternate Mind and Skillset for complex mental tasks. There should be some Cumulative Saves to represent the ongoing emotional stress of the Burn (Widsom or Charisma, Id say) and a much more difficult one if you actually touch the Shadow.

We know it's been asked a bunch whether a person can Bond multiple Spren and the answer is "Theoretically Yes," but has it ever been addressed whether a Spren can bond multiple actual Radiants (not just empower Squires) at the same time? Could a Spren Bond a Married couple, a set of Twins, or some other similarly Connected pair who share a spren and blade, etc?

I generally picture everyone with a little metal Startrek-style badge medallion (or spike) that Connects them to a bunch of shared Metalmind pools that everyone is using. But I think by the Space era they'll be able to convert one Metalmind's Investiture to another and Compound quite a bit so that a true 1:1 of each trait it no longer required. Imagine a shared life support system that uses Compounders to fill it and grants everyone aboard Oxygen, Nutrition, Hydration, Wakefulness, "Determination" (ie: carefully dosed mania for their duties), as well as the typical suite of mental and physical augmentation, healing, etc. just o make them generally better at them (either a universal package or task-specific specializations). Edit: also heat. And by that point they can survive it the vacuum of space which makes spaceship design a whole lot easier. They could literally swoop in on (metal) pirate ships with open decks, no pressure or atmosphere, etc required.

Precisely, and regardless of how the cracks are gained, the effects and the control granted to the Shards is a spectrum and a scale, so any source of emotional stress can increase the effect if it's significant enough for that individual to cause cracks. For example, it typically takes multiple spikes to allow Ruin significant control of an individual, but one spike plus a lot of emotional damage could probably do it too.

Fair point, though a person can gain Allomancy without Snapping, via Hemalurgy or Medallions. But regardless of whether it is natural or not, the spiritweb Cracks that represent Snapping make you increasingly vulnerable to influence by any Shard, traditional maddness is one such example, and there are degrees of it. So the question is whether Overuse will make somebody More than they were already (prior to or separate from the actual Savantism tipping point).

Sure, but causing Stress and causing Cracks are different things. It is Stressful to be confronted with an alternate version of yourself, including Thinking the alternate's Thoughts, and can potentially be very traumatic depending on your Choices and how much dissonance the differences cause. If the Stress is bad enough the Auger might break and then you'd have the Cracks, but it would be as individual as PTSD. Miles was content with himself (villainous as that may be), Vin too. But if a person who'd betrayed all their friends and nation and species and become a shards slave were faced with the version of themself who hadnt nuked their whole life, I could se it being quite traumatic indeed. As a completely random example.

Assuming it's inter enough to remain the correct Alloy, I think it would be harder to target with Intent than a single object that you can actually perceive and touch, etc, akin to [Stormlight].

You could be right, he's messed with the functional workings of the metallic arts in other ways per WOB. But WOB does say the loss of full Mistborn and full Feruchists is due to the interbreeding of those two Power sets, specifically, which didnt happen during the Final Empire where Feruchemist breeding was so closely controlled (and intentionally stunted). That's why Twinborn are a lot more common but neither full set pops up, and for the same reason Fullborn are not naturally possible (per WOB), the sDNA of the Power sets interfere with each other. RAFO on the rest, but you're going to love the ride.

Both examples there are seeing/manipulating specific types of external Connections, which is akin to Spiritwebs but manipulating them is pretty common and relatively low-power to accomplish. The Ghostbloods reference "Connection Tricks" as the typical way Worldhoppers (including Hoid) use to speak local languages, and the appear to be pretty common and accessible from lots of different worlds/magics. Direct Spiritweb Manipulation is generally on the upper extreme of Power requirements for the Cosmere, from what we've seen (likely because of the External/Internal divide so prevalent in the Metallic Arts). Aside from Soulforging which is a whole Magic system dedicated to it (and is temporary), Lerasium can do it, Bondsmiths can probably learn to do it, Hemalurgy can do it messily, and the Pre-shattering magic system called Microkinesis (which was Too OP for the Shards to allow to remain) could do it. Awakening is an oddball because it specializes in creating "counterfeit Spiritwebs" for objects. For the specific case of becoming an Allomancer, WOB says it takes both the Spiritweb change and a Connection to Preservation to provide the power, and specifically that Soulforging can do the first part but not the second. Lerasium does Both but it's stupid Powerful as a Godmetal and already innately Connected to the Shard. It's hard to say if any other powers we know of now have the juice to do both. And at the end of the day having the capability to Manipulate Connections and/or Spiritwebs and being able to Create new ones whole are two different things.

True, if Adonalsium were not ALL of Investiture in the Cosmere. Aether beliefs notwithstanding, WOB says it's all Adonalsium, and with it being an equal 3rd to Matter and Energy that doesnt leave a lot of wriggle room for them to BE the Cosmere itself (whether they created it or were Created AS it, Chicken vs Egg style). But there's also a concept of theological Urge vs Demiurge that WOB says matches pretty well (See Below).

There's an increasingly popular theory that the Shards themselves can be mapped to the same 16-slot grid that divides the metallic arts metals for Allomancy and Feruchemy. If true then it's likely that each of the 16 Godmetals will have some similar thematic leanings in terms of quadrant, Internal/External and Increasing/Decreasing. On top of that He retconned Atium specifically so that all Godmetals could be Burned by anyone without needing Misting/Mistborn, so the general intent certainly seems to be that the Metallic Arts functions will have reflections and implications in the broader Realmics.

I just meant that once you hit the speed of sound and are literally outrunning your own compression wave in the fluid (ie. rosharan air) a lot of the aerodynamic rules change entirely and it becomes a whole other rabbit hole.

Yes, they utilize the Surge of Transformation, per this WOB: The direct question was RAFO'd, but since RoW we can confirm that Surge Fabrials tend to be unBonded Radiant Spren. The theory since we saw them in Shadesmar is that each one uses an Inkspren and Lightspren as the two that Grant the Surge. We dont know how they were made but Im guessing it was Honor himself and probably around the same time as the Sibling themself since they are the heart of the gate system. Per this WOB, the Oathgates are specifically Spiritual Realm teleportation, distinct from any Cognitive Realm travel: Shardplate has a few differences with Shardblades, (No screaming, contented in their new form, etc). But I believe the issue is that Blades revert to their native spren form in Shadesmar, but with Platespren being "Content" in their new form, I dont think they retain the ability to transform back to their original Shadesmar/animal form.

It goes faster due to the elimination of friction, but it still faces inertial drag forces from the mass of air that is getting shoved out of the way as if goes by. As it speeds up and approaches the Speed of Sound (for Roshar's atmosphere), the friction matters less and the inertial forces become more prominent.

It's what they do. In my mind the scale is roughly "Died Before the Opening" to "Mufasa", with Bambi's Mom being a rough middle point.

Reckoners is really good but might be on the darker side of YA for a 10yo. It opens with some paternal death (not sure where it falls on the Disney Scale?) and then timeskips past the complete collapse of western civilization, so it had a dark feel to me just because it's mostly set in city-states ruled by supervillain despots but with places I recognized (rather than just being some fantasy setting or whatever). Anyone have an opinion on Frugal Wizard? I havent read it yet so I dont know where it falls on the maturity spectrum.

For the most part this issue only applies to things heavily Invested with an Individual Shard's Investiture; they are tied to the system/world which that Shard has Invested, and it can depend on the Shard's nature too. Endowment doesnt tie the Returned to Nalthis, but part of that is the nature of Endowment being a "No Strings Attached" sort of thing. Presumably a Shard that is not specifically Invested in a single System could invest a Being without it becoming Tied to a single System. But yes, as of his appearance in TLM he is the only Radiant that has figured out how to get around the typical Connection issue, per WOB. I doubt he is Bypassing Connections, but we know he's better than most at Manipulating them already.

Fortune should help find the right Command, given that WOB says it would help with things like spike placement and soulstamp development and the like. Mental Speed makes a lot of sense for exactly the reasons you describe. Blanking your Identity can make a stash of Breaths that anyone can access, although there's probably a specific Command that would do the same thing without the extra steps. I think you could make Tin do a lot in combination with the Heightening abilities but perhaps not with Awakening itself. There's probably a lot of potential synergy between the shapeshifting abilities of Returned (or Royal Locks) and feruchemy as well. I suspect you can store more F-Gold Health while you have Heightenings, since you get so sick upon loosing them. I suspect you could manipulate F-Nicrosil in strange ways to get more Investiture Per Breath out of things, or supercharge individual breaths so they can support larger Awakenings (that would normally take more for the same Command). I have a longshot hope that F-Cadmium will interact with Breaths, inhaled Stormlight, and other gaseous Investiture, but nothing really supports it beyond theme.

Ah, I missed the nuance, good catch.

"Immune" is a really strong term for it. It is technically possible but very difficult and not at all common, it would take a particularly strong and dedicated user. But even then, there are a lot of ways to pierce a coppercloud, both for individuals and for groups. EDIT: and as the WOB points out, a simple aluminum foil hat would do it a lot easier.

This line in particular from that WOB makes me think Vasher's (and Nightblood's) story will continue past SA book 10.

Ah, I stand corrected. Thanks! I could still see Discord influence and/or the cold war itself leading to a simple fracturing or nations, cultures, and ideologies, which in turn will create more diversity in how all three metallic arts will be used. War has historically good for giving bad people a supply of victims to experiment on, and a strong motivator to set aside morals for results. And that assumes no other Shards (or off-world refugees....) show up that bring their own flavor and mindset to the party. Hemalurgy is one of the rare magic systems anyone can learn to use anywhere, which Im sure will be enticing for many.