I have been to SD twice, HI once, Anaheim once. They were all great. Florida in August has always kept me away.
I am from Chicago. I work in suburbs, a mile away from a convention center that could easily host the event part with nice hotel, but visitors are stuck in suburbs with nothing to do outside of Pokemon. August here is somewhat better than Florida, but there are only a few venue's that I can think of that would be enjoyable/comparable experience to the others. The best venue that I could imagine would be the Chicago Sheraton that is downtown, right on the Chicago River and River walk, views, and walking distance to the Lake, Chicago Shopping District, and Navy Pier. From what I seen, the worlds location must have be in a tourist "destination"location such as the down town area. Saying this, I doubt that it would be Chicago or a urban destination like NYC.
Pokemon constraints has always been a resort location, with decent Airport accessability. I like SD and Anaheim locations in Southern Cal. I have no interest in SF. Seattle would be cool in August, but I think the Pokemon folks want to get away from Seattle area.
Small cities and non- "destination" locations have NO SHOT in this discussion. So the rotation of SD, Disney CA, Disney FL, and Hawaii is what we have. I could see Tampa being added to this list. The west coast destinations make sense to make it easier for West Coast people to make it, who can't make it east for US Nats.
BTW, Indianapolis for Nats was great. I think this should be stamped as a lock for now on. (Anyone hear anything to the contrary on this?)