It has been fun!
My campaign has already started. We've had a ton of party turnover due to life stuff, so the plot's a little screwed up anyway and I figured I might add this element I really like to compensate for it with cool factor. It might not work, but we've been having a lot of fun so far.
It makes sense for metalmind fill rate to depend a lot on metal, just from a balance perspective. Filling Copper might be an action, while filling Gold takes an hour.
The Ferrings I have in my party are an Archivist, who's metal is super easy. 1 charge = 1 page of text, once I figure out how many charges can fit into a single piece of metal I'm home free. Then I have a Bloodmaker. For her metal, I was thinking you can stop adding your CON mod to your hit point maximum for a period of time, and then spend charges to heal for your CON mod. Fairly straightforward, once I know how long it takes to store one charge. Then I have a Trueself. I honestly have no idea what to do for him. The dungeon I've made for the party to use as a base used to be a stronghold for Keepers, and so I'll let him use aluminum to mess with the Identity Locks in there. Otherwise I've got nothing. I also have either a Soulbearer or a Mistdrinker. Mistdrinker is the title for Adamantine Ferrings, because I made adamantine a God Metal. I'll probably make Nicrosilminds function as a ring of spell storing, but I'm not entirely sure what to do for adamantine. I want it to be similar to Nicrosil, but not the same. Maybe it can store spell slots or sorcery points? (The Soulbearer/Mistdrinker is a Wild Magic sorcerer, for reference.) My last party member is either a Windwhisper or a Skimmer. My struggle is that I'm having a hard time not making iron underpowered, but I'm also having a hard time not making tin overpowered (so far it works like a limited use Elven accuracy. The player I'm giving it to is very much a power gamer, so I'm worried he might find a way to abuse it.). Those are the metals I really need ideas for. Helmalurgy comes next. Allomancy is fairly unimportant to the plot, and someone else has already made Mistborn a D&D 5e class, so I'll probably borrow a lot from them. Thank you for responding!
I like limiting storage based on your relevant ability scores. I was doing it like Mistborn Adventure Game, but the system that it uses has easy exploits that make Bloodmakers and Steelrunners immortal without compounding, which isn't something I want. Especially because I have the amount that gold heals be based on CON, I like that the cap for storing is based on CON. Maybe Feruchemy could finally be something in D&D that cares about your raw score, not just your modifier!