Wisconsin Dells. It's a vacationy place, and I'm fairly sure it would have a convention center big enough for worlds. And it's in a new place and far away from hurricanes like florida.
One problem with the Dells is the lack of a good nearby commercial airport. The nearst one (Dane County Regional Airport in Madison) would probably have trouble accomodating that many people, and it doesn't have that many direct flights (mostly just to the Eastern hubs). The nearest major airport, Milwaukee Mitchell (probably on the same order as San Diego), is about two hours away. Granted, Kona's airport isn't exactly massive either, but this is worth considering.