My idea is to make a mastermind game with some interesting features, such as two-player mode, ranking, messaging, different levels....
Mastermind is basically a game that one player pick four digit and the other play guesses what the four digits are. Each time after a guess, a hint will be given in form "xAxB," where A means both the digit and the place are correct while B means just the digit is correct but not the place. This game can be more challenging and interesting while increasing the involving digits.

Anyone has any suggestion?