Why not make a game about this? Game mechanics are asking to be implemented themselves - arena fighting with a skull system, you start as a weakling, then, after each successful round you get a new skull, a new beast head! Or vice versa - a game about a person with a gun fighting monsters each getting more powerful and complex with each next round. It's got spirit of games like Binding of Isaac - nerve teasing but somewhat addicting!
Great job overall!