Create a simple, easy to use workout app that doesn't require a large time commitment to use. One that doesn't try to do too much: diet, lift, run, how much water you drink, how well you slept, etc. Basically be what Twitter is to Facebook, just do one thing very well. Focus on design as well as functionality.
Serious lifters carry around a book to track their lifts. I have an idea to eliminate that and also provide feedback that is in-depth. You should be able to track progress, what muscles you are working or not working, and recommend how to balance your workout to avoid injury.