First off, FF is unfortunately still open during busy seasons, so it's not empty. Not to mention that it's not a building, it's an open air theater, so they would have to knock it down and completely build a whole new building in order to put an attraction there. Thats more money than making Soundstage 44 up to code again.T
they have an empty area that is up to code (fear factor) that they need to get something in.
Not only do most people think they're probably going to put in a Despicable Me attraction there, but that wouldn't give them to much to work with, they would probably just use the exact same ride system, which, IMO, is awful.plus give up Jimmy Neutron (if license is expiring)