I don't know how doable it really is, but one thing I've noticed is conspicuously absent so far in the world of apple app music making is some kind of mic vocal preamps.
Since many of our instruments are coming into MultiTrack with effects already added, if there was any way to simulate a few of the most typical vocal mic preamps, that would probably be a huge selling point, as right now, my bet is that most mic signals are coming in "dry" with no real shape or coloring added. then again, I don't know how easy/difficult that would be.
Anyway, just thought I'd throw the suggestion out there. Rawk on, MultiTrack!