I have about a month to develop and write a small game in XNA. Problem is, I have absolutely no idea what to write. Anyone want to help me out with some ideas? Here are the constraints:

Game must have:

-A back story and setting/environment
-A graphical entity controlled by the player
-An autonomous entity (or entities) constantly in motion
-Interaction between the player’s entity and the autonomous entities
-Player must make an important choice (or choices) at some point in the game
-An additional creative gameplay element decided by you
-Sound effects providing feedback to the player

Anyway, I obviously can't do anything too complex, being one person with one month of time and lots of other commitments. I'm basically thinking of something akin to most flash games out there, in terms of scope.

Any ideas for me?