How about adding something that helps me to keep track of how I'm doing on puzzles over time. Kind of like a "records" feature that lets me know things like "fastest time to solve" and "percentage of spaces filled in".
I definitely have some ideas to move in this direction. I'd like Black Ink to be the app of choice for serious solvers - other thoughts I have are to print the solving time on the printout in case you want to keep a paper record of solved puzzles, etc.