Well, for Iron and Steel powers I see a solution.
When you are flying around in the city, a Spiderman-like system could be used. You don't exactly aim at what object you pull, you would just move "over there" and the character would automaticaly pull/push the metal objects around.
I also strongly suggest to not show the lines to the metal objects, but make these objects glow visibly while burning steel/iron. That would do the trick.
For a fight, well, I see only one possiblity - a combo system like in, for example, Enter the Matrix or even in mentioned Spiderman games. These combos would include all the tricks you can do in Mistborn world, including combos with Iron/Steel disarming, offensive Brass/Zinc attacks and stuff like that.
That's not the problem, if you don't want to have exact control of how the metal is used. But otherwise it's impossible.
And there's one problem I just noticed. If it would be a multiplayer game, we will have to say goodbye to Atium. I think it's obvious, why.