What's the motivation behind doing this exactly? Why not actually write some code to practice...writing code.
Despite that I can't imagine myself typing code just for the sake of typing code. Seems to me this would make more sense as a plugin for an IDE. Watch my every keystroke as I do real work and show where I am constantly having to correct myself, and offer ways to prevent that.
Either way, it could watch for bad habits and common mistakes, show aggregating stats, etc.