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The Monster Island games, the rules are fairly simple, you can make a monster in minutes, each of the three books (the other two being escape from MI & Invasion of MI) comes with more to add to the game, with battles in cities against the military, against the attacking aliens, in a 3 way battle or even wargames with monsters.
There's extra monster ideas from the website (they give you a lot of pre-made ones, like Godzilla re-named, Mothra re-named and even Dr Manhattan!) and there's enough powers to make just about anything, I could run an Evangelion game with it in theory...
Was tempted to make a board-game spin on it with changing weather, events (power crystals, changes to game mechanics, excuse to create a monster from an area) and happenings from staying in certain areas.