What I want is simple: I want Allomancy to feel right. I want it to feel graceful and theatrical and spontaneous, something where the limit really is your imagination and not the ruleset. As part of that, I think the combat system must be fast; nothing kills the feeling of tense combat like it taking 10 minutes to get back to your turn.
That's the big one for me. As minor requests, I'd like all the major "types" to be equally viable as PCs (misting, mistborn, ferruchemist, kanda, and normal human. Inquisitors are bonus); I'd like atium to be scary but useable (as in, you should fear anyone who has some of it to burn, but it should be possible to get and use some of it yourself, even if it only happens once per campaign); and I want there to be cool artwork, because it just makes the book so much more enjoyable to read.