I think what's missing on most "piano-games" is that it doesn't really mirror the way I learn a piece.

The way I learn is by bars, repeatedly re-playing bars say, 1 to 4, until I get them, them 3 to 7, until I get them, etc.. But most piano apps want you to start playing from start to end slow, then faster, etc.

So: piano apps should allow breaking a piece into bars, allow you to repeat those bars again and again until you exceed certain score. Maybe even keep track of which bars are harder to play, and repeating the difficult bars more often, etc.