The game at Expert difficulty level seems a bit not that hard except the maps become bigger. I can think of a few points below at which the game could become harder:

- Avoid prime or even odd numbers. They make variations less.

- Avoid very large numbers. They basically just make the maps smaller.

- Make the numbers apart so they don't interfere much with the variations of each other. (This might require more skills to be put into the development.)

@patheros Thanks for the feedback. I will try to improve the puzzle generation. Right now it is kind of basic. It just creates random rectangles in a loop and tries to fit it on the board.

@kwikc Great suggestions, I will look into the puzzle generation and try your suggestions.