I just discovered this site today, so I'm new here.
My idea of this would have a campaign and multiplayer, where you play as a mistborn in campaign and have many different modes in multiplayer (such as mistborn free for all's, maybe a co-op of mistings, and surviving waves of hazekillers playing as a mistborn). The metal burning could be assigned to your d-pad, and change when you press your left trigger, as such:
COMBAT LAYOUT
^ iron/steel
< pewter/tin
> chromium/atium
v duraluminum/drink vial
Here's a chart of the 18 metals, just in case
http://www.brandonsanderson.com/graphics/mb_table_v13.jpgsomething like that. the atium in multiplayer obviously cant show u where people are about to move, so when you burn it you will dodge every attack and yours cant be blocked. due to the extremely OP nature of this, you will have a very limited amount of atium per multiplayer round, if its even enabled, and drinking a metal vial will not replenish atium.
Anyway, just a few ideas. I'd love to see something like this happen