There is a chess site I used to play at, but stopped because I discovered RHP, that offered variations on chess. I don't know how hard it might be to adapt the current code, but I quite liked some of them, and they bring a different challenge to traditional chess.
There were things like suicide chess (where the object is to lose all your pieces), horde chess (in which one side has a normal setup and the other has 4 rows of pawns), and quite a few others that I forget at the moment.