DO NOT TRY TO MAKE A GAME THAT LOOKS ANYTHING LIKE THE ONES THAT ARE ALREADY OUT THERE. That's the best advice I can give I guess. People will never leave a game they are comfortable with to play another one that is almost the same thing. You should also have some comedy in your game and try to make the setting original (No orcs, goblins, etc), see Dungeons of Dredmor.