Construction – covers a wide variety of game types: (eg, Solomon’s Key, TRI, Minecraft, Tower Defense) Construction can be for offense, defense, art, and puzzle solving, or just for the pleasure of tech trees.
End of the World – Sets the scene for a myriad different background stories. eg, Alien invasion. zombie apocalypse, eco-awareness games, exodus from earth.
Ghosts – Haunted house games. Or you are the ghost doing the haunting. Or it can mean doing away with collision detection, which can open up lots of possibilities for less experienced programmers.
Guardian – A boss encounter. Or you are the guardian protecting something from wave after wave of creeps.
Inverse World – Upside down platformers? Things replaced by their opposites? I dislike this theme.
Mirrors – Laser games. Mirrors in a PvP sense of like vs like. Mirrors as portals (eg, Alice in Wonderland).
Outer Space – Lots of space games possibilities. Or more abstractly in contrast with an Inner space, it can emphasise the periphery of the game world.
Quarantine – Things escaping? Trying to manage a contagion? Not too many interesting ideas for me here.
Salvage – The idea that in game items can be sold or recycled. Quantity over quality.
Symbiosis – Interesting gameplay possibilities. Multiple player controlled entities which support and complement each other.
Trapped – Fairly generic theme which somewhat resembles the Escape theme from LD21. Dungeon crawler? Jail brake? Stuck in a well? Maze?
Unstoppable – Runaway train concept. Or a game that never quite manages to finish.