There IS a fifth park. Its called Universal Studio's
Wouldn't that be a fifth AND sixth park? And what about SeaWorld?
Seriously though, I actually agree with the OP; we need a new park. I absolutely understand why that is not happening, and I agree that there is room for expansion at other parks, but I also think there is a reason that Uncle Walt bought so much land. Yes, a big part of that reason was so that he could build his own resorts for guests to stay at without having to break the Disney magic, but I don't think he envisioned building so many resorts that the capacity ratio skewed in their favor. Of course, we'll never know what he would say.
I don't think that building a new park and expanding current parks are mutually exclusive notions. Definitely there are some good opportunities for expansion at MGM (I'm sorry, that's what I'm calling it). I love the idea that CentralFloridian99 raised, that MGM should build an entire Star Wars "land." If they can get G. Lucas to sign off on that, they absolutely should. I've always wanted to go for Star Wars weekends, but haven't yet had the opportunity.
I read recently on another thread a suggestion that they should build a Lord of the Rings/Middle Earth park or land. I know Disney doesn't have the rights to that property, but if they could get the Tolkien family's blessing, that would be possibly the greatest thing ever. Regarding licensing for the film franchise, I can imagine that would be feasible. After all, it's a Warner Bros. property, and they are free agents in the theme park business. They already license their cartoon properties to Six Flags, and the Harry Potter films, obviously, are represented at IoA. Plus, if I'm not mistaken, WB has multiple properties represented in the Great Movie Ride. Casablanca, at least, but I think there are others. And really, you could easily build an entire park around Middle Earth.
Apart from the business and financial reasons, I think another big hurdle to Disney's building a new park is the fact that it would need to stand apart from the existing parks. Of course, there is always that LotR idea, but I expect Disney would rather do something with broader appeal if they were building a new park. The problem is that most good ideas for attractions could already fit right in at one of the other parks. For example, I could see where it would be a good idea for Disney to build their own version of Sea World, but then it would make a lot of sense to just add that on to Animal Kingdom, especially considering that AK is in need of some new attractions anyway.
Maybe now that they've closed ToonTown Fair, they could take all the classic Disney characters and give them the royal treatment with a park of their own. Give Mickey and Minnie and the rest of the gang their own houses again, but make them bigger. Maybe build a restaurant in one, even. Build a Fantasia dark ride, a la Peter Pan's Flight or the Scary Adventures of Snow White. Stage an animatronic version of "Conductor Mickey" (Yes, I know that already has representation in Philharmagic). Steamboat Willie could easily be made into a boat ride. I have to say, I see some potential here

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