I love mastermind also. I could see a few different ways it could be brought to a game site.

1) opponent vs. opponent - that is each person picks the color sequinse for the other player to figure out. If you figure yours out before the other player figures out the one you made for them - you win.

2) System picks a random one for both players to try to figure out - one to do it in less moves wins (can not see what the other player is guessing)

3) The system picks a random one, then each player takes turn trying to solve the same puzzle - so you build off of what your opponent does. That is player 1 playes. Player 2 sees the results, and tries the next guess. and so on.... even though while I type this one, I can see some just stalling forever so they don't give their opponent any help, so this one might now work.

I would also recomend that at least 5 codes are used (I believe the 4 has been solved in like 5 moves) - 6 would be best. (Or have 2 versions - a 6 long, and ultimate 8 long or something)