Recently, I was required to develop a game as part of an interactive computer graphics course. I kept having ideas like "tetris on steroids" and "bejeweled meets ricochet", so I decided to hack together an automated system to generate thousands of such ideas. The result was ultimately completely unhelpful in relation to the project, but has given me a few good laughs with its ridiculous suggestions:
The multiplayer of Gears of War meets the art design of Mario Kart Wii.
The next generation of Golf.
...that last one could get rather ugly. If you want, you can give it a shot for yourself: http://oddfinch.com/gameideas/
The controls of Grand Theft Auto IV meets the characters of Nintendogs.
Let me know if you find anything awry, but note that unfathomably terrible game ideas are a feature, not a bug.