What would you want from a "practice assistant" app?

[coherence]

I'm working on a mobile app to help me practice more effectively.

It turns out trying to learn The People United Will Never Be Defeated haphazardly, without a teacher, is slow going for a busy amateur like me. So I've been looking into ways to use my practice time better. I started out with a technology-free approach, just trying to focus better, avoid practicing mistakes, etc... but I've since figured out a few ways to use my phone like a tutor to keep me on track.

I'm interested in fresh ideas, so I won't get into what I've tried so far. If you had a phone (or tablet) up on the piano next to your music, what do you think is the best thing it could be doing for you? (Or if you're a teacher, what would you have it do for your students?)