There's other stuff I want to see that should be even more 
  interesting. When it's replaying, I want to be able to see 
  characters that make it into the final version in one 
  color, and others in another. Then I'll be able to tell 
  when I'm writing well and when I'm writing badly.

How about coloring text based on their age in the work? That is, characters added in the last edit are bright blue (or green), and as they age without changing, they fade to black. That would be extremely cool to watch.