Here are some ideas to add on to t4u's.
After the end - The afterlife
Death is useful - Necromancy
One rule - A dictator or monarchy. A "lord of the rings" themed game
Playing both sides - Spy game. A two player game (like pong) where you have to play as both and keep switching every 10 seconds
You are not supposed to be here - Spy game for this one also
Deep space - It does not have to specifically mean flying a space ship, this could mean isolation being away from family/friends/home
Deja vu - You can make a game where you keep playing the same board, but changes you made previously stay there
Entire Game on one screen - It is screen, not scene. That means no camera motion and no scene switching. You are free to make any type of game, but figure out how to keep everything in one place.
Snowman - I don't think you have to make a game specifically about snowmen. When you look at it, think about what it reminds you of. To me a snowman reminds me of the season, playing, or even rolling a ball around. You could make a katamari damacy game where a ball keeps getting bigger and bigger.
I left off some of the themes because I am drawing a blank on them.
edit: oh, avoid the light - A stealth game. A game where enemy light radius triggers it to attack (not specifically stealth related)
