If I were programming it, I would need to do it on the computer.. Consoles just don't have the buttons to allow the flexibility that I'd like to see in a system like this...
Basically, I would set it up like a First Person game, since they would allow for more versatility in the movements and actions... Mouse control to move the sight... Right click would lock viewpoint and make the mouse move within the current viewpoint for the sake of selecting a given 'strand' when steel or iron are activated... Selection of the powers would be done via hotkeys, with flares controlled by hitting it twice in rapid succession...
Any metallic object would be given a variable that would indicate its weight, and when fed in an appropriate algorithm, would equate to a semi-translucent bluish beam of relative width when you have Iron or Steel activated... Atium would be done with a bullet-time like system where a semi-translucent shade would be the actual form, and the real would just be a second or two behind it, but capable of being killed..
Emotional Allomancy could be used both in conversations and possibly as a sort of 'jedi mind trick' style thing, where you could increase / decrease the hostility of someone... Imagine the fun of being a rioter and just sitting back and making a guard freak out on a guy for no reason to get into some place without having to kill someone...
Pewter would both increase your attack damage, reduce your damage taken, and when flared, allow you to continue playing even after your life / hp / stamina / whatever is used is depleted... Decrease / eliminate your chances of falling while balanced precariously or something as well...
Hmmmm, the more I think about it, the more I believe it would be a fairly complicated system to actually play effectively, unless you dumbed it down... But the potential fun that you could have in a sandbox like design would make it well worth it, in my opinion... A lot of the concepts would be fairly easily represented, the only thing that I think would prove a challenge for a game designer would be to make zooming through the air, and effective pushing / pulling into a fun thing... I would definitely do it as a first person game though, I don't think that anything else could do the combat / air travel justice.