My idea for how a Mistborn MMO could work:
Have the main setting in Luthadel
Make the players mistings in thieving groups
Have raids be on keeps of various influence and power, and the "wilderness" be city streets, with guards.
Have XP be in terms of either gold stolen, or notoriety gained.
A successful group would require more than just coinshots and thugs, balanced below, but might be limited to first four.
-Coinshots: mainly fighting class. Steel could be controlled with "push objects away from self" and "push self away from objects" selector, with bracing yourself and not being able to push off of light objects handwaved away.
-Lurchers: Counters coinshots. Could be given better mobility than them, as well as the ability to pull teammates precisely.
-Thugs: Pewter's poisonous effects could be played way up, as well as how fast it burns, to make pewter mostly viable only while fighting already.
-Tineyes: Scouting, maybe giving increased information on the enemy, including better critical hit chance (?) and accuracy with bows.
-Soother: Debuff enemies with apathy, buff friends with focus.
-Rioter: Debuff enemies with fear, buff allies with courage.
-Seeker: finds enemies, and IDs them.
-Smoker: hides from seekers. Immune to debuffs, might give resistance/immunity to teammates as well to make more useful.