Havent read through the whole thread yet, and have absolutely NO experience with designing/programming games, but heres my 2 cents. I think it would be best as an FPS-like mmorpg, for the computer.
Zinc/Brass I think that it would be cool to have different emotions cause different effects, like debuffs (i.e., rioting fear causes your target to panic, and run away), in addition to dialogue scenes.
Iron/Steel I think that the "bubble" idea (you can push, pull on everything in a certain radius) would be cool, and as for being a mistborn/inquisitor, and burning both at once, the lines could b different colors.
Each metal/feruchemical ability could be a different skill, and level up individually. You could start out as a misting (you choose the metal you start with). There could also be a system that determines which class you are at a later level, feruchemist, inquisitor or mistborn, like in Spore (inquisitor is "evil" path, mistborn is "good" path, and feruchemist is "balanced" path). You could buy or find the metals in the cities, and find coins lying in the streets. You could also buy horseshoes or other "anchors" at stores.