MGM just doesn't really have the room to expand. World Drive on one side, Buena Vista Parkway on the other, Parking Lot on the 3rd, and the main entrance on the 4th. There are also a lot of backstage areas which are used that limit it's ability to be further expanded. Plus, The park is next to some conservation areas which further limit it's ability to be expanded. There is some land nearby designated on the land use diagrams for theme park use, but it's on the other side of world drive. That would require a bridge to cross over the traffic to give you access to it from MGM. (Tunnel wouldn't work because of Florida's high water table)
Blue sky idea:
Enclosed tunnel/attraction, over the road, into the new theme park space, providing 2 way transportation between the two areas.
Yeah, I know...bottleneck and all that. Don't kill my blue sky with logistics.
Seriously, though....unless they took over the back lot tour/sets area (which they could, I guess...if the rumors about more Pixar aren't true) AND maybe the Indy show and Stunt show areas....they're sort of boxed in.
The one thing they COULD do is swap the conservation area for other undeveloped land earmarked for something else. They've done this before (most recently, I think, with Golden Oaks?) so it's not out of the question. But it's probably extremely unlikely.
The Magic Kingdom doesn't have the room to expand outward due to service areas and service roads.
Blue sky idea: Fill in parts of Bay lake and start expanding outward!!
Yeah, it would be a nightmare. I SAID don't kill my blue sky with logistics!!
Seriously, though...any MK expansion is likely going to have to happen within it's current footprint for exactly the reasons you mention. At least for a good long time. They have pieces (the diorama space around the train ride, for example), but nothing big enough for large scale expansion. Maybe, eventually, technology will provide an answer (shrinking some of the facility and infrastructure space required) that gives them SOME more room.
They could also, probably, do SOMETHING with Tom Sawyer Island and the river around it. Not easily, but possible...there are those crummy logistics, again. And ME would be a HORRIBLE fit between (most obviously) Frontierland and Liberty Square.
Ultimately, I agree with your sentiment that it's not likely any time soon.
Animal Kingdom also doesn't have easy access to further expansion areas beyond the beast kingdom plot currently partcially occupied by Camp Minnie Mickey.
Blue sky idea: Knock down the facilities between AK and AKL, fill in the swamp land and evict the crocs, and expand there. Oh, and build a walkway between AK and AKL while you're at it (with a jogging trail and bike rental), with a completely independent gate into the park!
Yeah, I know....stinkin' logistics!
Back to being serious: To be clear, even with FOTLK's theater, Camp Minnie Mickey only takes up a portion of the Beastly Kingdom "pad". Now...I can't remember if Avatarland is set to use another portion of that space or not. If so...there's very little space left. If not...you have a good sized pad (because you rip out Camp Minnie Mickey, entirely) to use.
They could also rip out Rafiki's planet watch/Conservation Station, whatever and use that space...but it's not all that large. I guess if you rip out the train ride back and repurpose ALL that space, you have a bit more.
In all, AK probably has the best potential....but isn't a great thematic fit (especially considering Avatarland is already slated to go there...you're turning AK into another movie park). Once again, I'm with you on "unlikely".
EPCOT also goes without saying.... no expansion room due to EPCOT Center Drive, World Drive, The EPCOT Resort Area, Buena Vista Parkway, and all the service buildings for the park.
And of course.... none of this touches upon the fact that middle earth just wouldn't fit within any of the current parks thematically even if hell did manage to freeze over resulting in Disney gaining the rights.
Blue Sky idea: Build an expansion under water, in the lagoon.
All right! ENOUGH WITH LOGISTICS!!! I don't care about %^@#$ Logistics!!
And a final return to serious: Epcot has, reportedly, a few open pads for use....and you could rip down the old Millenium Celebration pavillion (though it actually gets used for special events like food and wine) and expand back toward the UK. There's some room there, to use...but not much of it is contiguous, which means you're stuck in terms of size.
It's also a HORRIBLE thematic fit for World Showcase...and not any better for Future World (unless you're thinking that Middle Earth is a great depiction of what Earth will become...personally I'd be more on board with a "Zombie Apocalypse themed ride).
So, finally, we're right back to "I agree, unlikely".
I think Disney was looking for SOMETHING to go into AK a couple years back. I think the ME surveys were probably the precursor to what would ultimately be Avatarland. Disney decided to grab a different IP to fill the same role.