So here's a basic idea. Characters are placed in modern day Earth, absolutely no different to ours up until the moment all these other worldly people start turning up and messing stuff up.

No jamjar, no power limitation and game plots would only be in response to what characters do. The world info will change to reflect everything that occurs and players are encouraged to seriously alter the gamescape. To help things along in this manner, I'd personally play a villain who'd not change his goal to conquer the world, but it wouldn't be treated as a mod enforced plot and sure as hell wouldn't be rail roaded with a foregone conclusion.