Now, this has to be reasonable. Don't just copy a game that has already been released, and don't tnink up a game that is not possible with current technology etc. Be original and be informative.
What would the name of the game be, what would the single player (if any) be like, what would be the story or plot to the game. Will there be multiplayer, if so, what sort of game modes would their be? Team Deathmatch, CTF? or an original gamemode you've thought up.
If you really want to go into detail, what would be the core mechanics, eg. a steal game, assassination game, first-person shooter, etc.