Perhaps you could add in reverse mode. Reverse mode would only be unlocked after the completion of all 100 puzzles in the original mode. In reverse mode, players will randomly be given a puzzle (random puzzle of random size) and in this mode they will have to do the exact opposite of the puzzle. The grid will start off a black with white lines through it (marking the individual squares) and players will have to fillin the white/null blocks to show where the black coloring should not be in order for the puzzle to end up looking the exact same way it did in the first section.
It’s a really great idea. If sales pick up this will be one of the features we will add.