The AvP idea is actually bad. It recquires too much strict rule following and character progression, which I feel would be difficult for a bigger amount of people (3 and up) could follow through with everyone posting.
I can, however, Run a Giant Monsters RPG. A Godzilla RPG. You could choose what type of genetics you are, like Mutated, Natural, Alien, Magical, ETC and you could choose two base powers and 3 abilities like flight or laser eyes, and like base powers, flame, ice. So you're laser eye ability could shoot Flame or ice.
The more worhsippers you get the stronger you become, to the point if you're... lets say a giant natural originated Crab with huge pincers and claws, you'd develop spikes on you're body, ETC.