I'm not exactly sure about all of the details, but I have heard (and been to the location) that six flags once owned a water park that went out of business just north of Miami. I think it was actually in Diana Beach, but don't hold me too it. Now, it's a place called Boomers with stuff like an arcade, go cart tracks, mini golf, and stuff like that. Right next door is a big wooden roller coaster named the Hurricane that you could see from 95. In front of the really large go cart track, there is a submarine sitting there. Supposivly, this is from the Six Flags water park.
I can't see Six flags building a park in orlando because it's not likely they could compete with Disney and Universal unless they get something totally new and well themed. South Florida would be an ideal location, but if they already had a park there that failed, I can't see them building another one.
I think Wild Adventures buying Cypress Gardens and adding their talked about boardwalk complete with roller coasters is the best bet for the next theme park.