Third time trying to post this.. *crosses fingers*
I would like a sandbox styled RPG, with a twist.
Open world and non-linear, but with ceratin anchor events that unfold in the story arc, you switch over to another PC. Sort of like LoK: Defiance. I think it would lend itself to Brandons character development quite well. Additionally, this way you can play a mistborn, feruchemist or hemalurgist in a playthrough, but it isn't necessary for them to essentially write three serparate games as they would have o do if you were only able to pick one of those "races".
Though, I would be happy with just having a single mistborn PC.
My hopes would be that it is real time and not turn based in order to really capture flying through mists (if the setting allows) the effects of atium and so on. But, so long as there is a game, I'll be happy, I think.
I like Knightmare's ideas for the different allomancy skills! Perhaps, in addition to leveling those skills, some metals don't become available right away. In addition, there could be metal vendors of differing reputation and skill. Some perhaps aren't as good with the purity of their metals or in making their alloys. Depending on who you buy metals from can determine the efectiveness of allomantic skill. This could be a good way to scale how powerful you are.