My suggestions are: - A Follower - Weapons are Resources - Bigger may not be Better

The way I chose mine was by trying to be as vague as possible. That sounds weird, I know, but being vague lets people interpret what the theme is. For example, my theme A Follower could be interpreted as: - A friend who follows you - An enemy or obstacle that chases you - A group of cultists or otherwise who follow your decisions - etc.

Basically, vague = open for interpretation, which is good, because it leads to much more diverse game genres and ideas, rather than "oh the theme is December let's make a game about Santa and snow"