Thank you for the feedback!
An extra-hard mode would be cool indeed. It would require me to improve significantly the AI though, as hard mode is already unleashing all current logic can do. I definitely add it to my TODO-list, thanks for suggesting it.
An arcade mode is in the work, with a timer so you can improve your score each time, and possibly opponents with unfair advantages.
Oh, and did you try the Konami code on the title page?