I've got a pretty big, overarching theory about what's happening in the Cosmere based on Biblical parallels and would like some other opinions. I also uploaded this to GitHub if you're interested in contributing or opening issues, but for some reason the spam filter won't let me post the link.
TL;DR
So, the TL;DR of it is that I think 1) Adonalsium was not the God itself, but a component, 2) that the God associated with Adonalsium has it's powers hidden under the Shattered Plains, and that 3) that is the pool of which Mishram drank and why she can threaten Odium himself. I'll also throw info about how all of the aspects of the gods (vessel, shard, metal/well of power) we see in the Cosmere map directly back to the Christian God (via the Trinity and transubstantiation) -- but I won't go into too many details, I just think it's cool and painfully obvious once you spot it.
There will be a postscript with wild speculations about the Dawn, from which the Dawnshards come -- and where my theorizing is prolly off a bit. Needless to say, the Dawnsingers might be called Dawnsingers because they were created to Herald in the Dawn of a new era of the Cosmere. This also has Christian undertones, which we'll get to.
I'm not overstating when I say that, in my reading, everything, from the structure of Godhood in the Cosmere to the Shattering of Adonalsium -- even why Hoid thinks his quest will help bring his old friend back to life -- are nearly direct parallels to Trinitarian Christianity.
I'm not Mormon so I can't pick out those details, but I would absolutely love if someone who is Mormon can give me their read on any Mormon-specific subtext I'm missing here.
Structural notes
I'll start with 3) and work backwards, as 2) and 3) are more directly related to WaT and less speculative than 1), but from here on out I will refer to the singular God of the Cosmere (the one associated with Adonalsium and, arguably, the one Dalinar and Vo [the Austre guy from Warbreaker] both interacted with) as "Creation" just to reduce ambiguity between Adonalsium (the God metal/body) and Creation (the power/Shard) going forwards.
Also, presumably the vessel is named Adonai (you know, the Hebrew word for "Lord" that Jews use for God), but that's not relevant here. This combination is what I think the "Seventeenth Shard" that Frost's organization calls itself is referring to -- the original One God of Creation, which Frost believes will eventually come back if the current Shards are left alone to experience life on their own.
Mishram drank the blood of Christ (not Odium) (slurp slurp)
We all know Mishram drank from a well of power and we're led to believe it was Odium's well -- but, how would drinking Odium's power give Mishram power over the sapiant Spren? We know those are of Honor alone (well, one or two also of Cultivation). Mishram's imprisoment made deadeye spren, but Odium only has power over Unmade spren (which is why they're called Unmade, Odium jacked 'em up after he showed up from Ashyn). However, Mishram could also grant Singers forms of power associated with Odium. How could Mishram gain a hint of the power of both Honor and Odium?
It was said that Mishram's power was a direct threat to Odium itself, but how could that be if Mishram drank only of Odium's power? Nowhere else in the Cosmere do we see people able to threaten the Shards themselves with a little bit of their own power -- we've seem many instances in the Cosmere of individuals taking and using from various wells of power and they never once became a rival to a god. Odium's power thirsts after Mishram -- and we are led to believe it's due to her nature, but we also know that Mishram was actually very honorable and willing to set aside her passions and make a deal with the humans. Not very odiuous to me. Rather, I think Odium's power thirsts after Mishram for the same reason the Dawnshards are uncontrollably drawn to each other -- they are of the same power and they want to be one.
No no no, Mishram didn't drink of Odium's pool, Mishram drank of the ultimate power of the Cosmere -- the well of power associated directly with Creation and Adonalsium. The well of power that came from the Original creator of Roshar, the well of power from which both Honor and Odium derive.
Also, ask yourself why Adonalsium and Ba-Ado-Mishram both have "Ado" in their name? That's not a coincidence, that's a hint -- and Brandon loves that ish. He's sticking your face in it some you can come back and smack yourself for not seeing it.
Where the blood at tho
My guess is that Creation's well of power is hidden under the Shattered Plains. When Honor and Odium clashed, the shattered plains split open and Honor saw:
I saw people there -- new ones, watchers, who had hidden from me. Those pieces of the sky...they sheltered from the eyes of God? That was not aluminum. It was something greater. Something...that responded to our clash, the ground liquefying in a pattern, dictated by the ton and the strange nature of the place.
Guardians hidden from the eyes of Honor and Odium themselves? Hidden by pieces of the broken moon that shattered the Shattered Plains? Power that responds to the clash of Shard's themselves? Golly gee, I wonder what could be that powerful?
I think you all see where I'm going with this, but -- to me, at least -- it seems like maybe the remnant of Creation's well of power was stored on the moon that crashed into Roshar. I believe that Odium, after seeing this during the conflict, then decided to place his well of power above this hidden power to hide what was truly there.
We also know that the moon that crashed into the Shattered Plains crashed there before Honor and Cultivation showed up to Roshar, so if I was a betting man I'd guess that the moon shattered the plains when the 16 shattered Adonalsium (see the poetic nature of that -- Brandon loves ish like that too).
Adonalsium ain't a God, bruh -- and the shattering of Adonalsium was just big boy version of Wax shattering Harmonium
I think it's clear from the name that Adonalsium is a "God metal" and not a God, let's be real here. When I first got into the Cosmere, I thought it was so weird that Brandon would take an obvious reference to the biblical God and slap "-sium", which just seems tacky to me -- but now, I don't think that's the case. It's a misdirection, there is an actual God/Shard held by a being named Adonai and Adonalsium is no more the God than Atium was Ruin. I think this is fairly well establish in Cosmere lore by this point, so I won't harp on it.
I've referenced this above, but just for clarity sake I will break down the exact relationship between the Trinity and the God/gods of the Cosmere. Each God/god has three components that map to the Trinity and one subcomponent that maps to Christ. Additionally, these three Triune components map to the three realms of the Cosmere:
The Shard
This is the abstraction of a God/god's power
Represented in the spiritual realm
This God, yo
The Vessel
This is the wielder of the power which influences the world
Represented in the cognitive realm
This the Holy Spirit, yo
The Physical Manifestation (please give me a better name for this, I'm too dumb to come up with something)
This is the physical manifestation of the power
Represented in the physical realm
The Physical Manifestation is actually two components (the Bread and Wine of Transubstantiation):
The Body - this is what we normally call the God metal (which has, not coincidentally, also been referred to as a God's body in previous books)
The Blood - this is the well of power
This is Christ, yo.
You will, however, notice that this is just the body and blood of Christ and not all of Christ. I think Christ is fully "the Dawn."
Given the above breakdown, what I think happened during the shattering of Adonalsium was basically just like what Wax did in Mistborn Era 2 -- they shattered the body of Creation (Adonalsium, it's God metal) into 16 components that they could consume to gain god-like powers. Wax split Harmonium into Lerasium and Atium -- 2 components that could give even smaller god-like powers.
Wit is very clear in WaT that the Shards aren't actually Gods, they are gods -- beings of great powers, but not God God. Just like Mistborn seem to be Gods to the South Scadrians, but are just peeps with cool powers to those in the Basin.
However, that means that the Vessel, the Shard, and the Well of Power of Creation are still out there influencing the Cosmere (shocking, I know, who would've guessed that Dalinar wasn't crazy when he was talking to the monotheistic God in his visions).
Random Dawnshard ish, I dunno
We still don't know much about the Dawnshards, but we know this: the Dawnshards were obviously split from a larger whole and wants to combine back into it's original self. We saw that in WaT with Rhys and Wit, they were uncontrollably drawn to each other.
The fact that the Dawnshards came from (waves hands wildly) something called the Dawn, how the absolute eff does that relate to the Dawnsingers? And also, how does it relate to Creation itself?
My huge, unfounded guess is that the Dawnsingers are called the Dawnsingers because will Herald the world into a new Dawn, one where the power of Creation knowingly allowed it's Body (Adonalsium) to be sacrificed and split into 16 parts in order to truly understand the universe it created. The Dawnsingers are the Christians in this story, to be the followers of Creation after Adonalsium's shattering (crucifixion). Guaranteed they will be "in communion" with Adonalsium in the future, drinking from his well of power -- just like Mishram, Ado's first Acolyte to drink of his blood.
The guess, really, is that Creation had planned the shattering of his physical body long before Hoid and his ragtag band of weirdos even thought of shattering Adonalsium. Creation created an entire planetary system designed to carefully drive the shattered powers back to him over time, after each of the powers had experienced the universe and matured enough to be back together and usher in salvation -- you don't think Dalinar was just saying that the power of Honor had to mature a bit right before Dalinar gave the power up to Teravangian? Dalinar saw what Creation wanted -- what Creation was prodding him to do -- in that moment.
The Dawn is the tool created by Creation to Dawn in this new era and the Dawnsingers were the the people he created to sing the songs of the new era. Specifically, I think the Dawn is a representation of Christ as well. This sounds really weird until you realize that Christ sacrificed his body to bring about Salvation. So Adonalsium == The Body of Christ and the Dawn == Christ -- so, just like Jesus sacrificed his own body, the Dawn splintered itself into the Dawnshards to sacrifice Adonalsium.
Seriously, why the frick would a being as powerful as Creation give Hoid of all people such a power? How does Hoid have enough Fortune to always know where to go? Hoid is another tool, just like the Dawn and the Dawnsingers, of Creation -- although, ironically enough, I think Hoid thinks he's the one pulling strings.
This story really seems to parallel the mythology of the Iriali -- which makes it odd that they'd leave Roshar at the end of Era 1 SA and head to Scadrial just in time for the new clash of the Gods, eh??
My big question here is what from the biblical record do the Dawnshards correspond to? I'm absolutely certain that they correspond to something, but I'm not about to reread the entire New Testament to try to find the relevant "thing" that is split in four parts.
The Aethers
I dunno bro. I'm still trying figure out their place in this. The one thing I will say, though, is that given the fact that the Iriali where on the Aether-fricked planet of Lumar (Tress' world) during their journey makes me think the Aethers have a hand to play here too. Maybe they're the Pagan Gods that clash with Creation in all this, I dunno. They do say they were here first.
Final Thoughts
Adonalsium's shattering mirrors the crucifixion of Jesus and when Adonalsium finally returns it will usher in the end of days and the dead will rise from the Beyond -- this is why Hoid is so busy interfering, God stole someone he loves and he wants to bring them back from the dead (we hear this directly from Frost in The Traveler).
(also, I lied, I'mma boutta reread the frick out of the bible to try to figure this out lmao)