One of our original ideas/themes for the runner game was to have the player character be chased by something. Whatever was chasing the player could influence or create the whole theme of the game itself. We thought of an avalanche, so our player could be a little eskimo, or a rising tidal wave, where our player could be a surfer dude. Eventually we got into the idea of the player being a little egg, and the force chasing it was an army of....sperm.
We found that to be kinda awkward, but we did like the idea of some slick little runner woman hauling ass across the level, so we went with it.
