I am making a game... A very simple game, and I realized I can fit thousands of themes on it.
So, let's take a existing game as example: Ant Smasher for iphone.
It is like any other rail shooter: Thing pop on screen, you click/tap/shoot/whatever it. And it dies.
You can make that ant smashing, zombie shooting, alien exploding, ladybug scaring, devil smithing, and so on...
So, when you have that great game design that you can fit a thousand themes, how you decide them? Specially if you are trying to choose the one that will be most popular.