Quick idea: Perhaps some functionality which pays attention to which music you play during your run. At the end of the run, let the runner "grade" how they feel after their run. Help the user generate playlists for future runs based on this data. (Although, after typing this, it sounds gimmicky and probably doesn't work well for the hardcore runner audience which probably want a clean and simple app with a small set of strong features than a bunch of weak one. I guess it depends on what your app does.)