I took a look at the maps. I'm not any sort of expert regarding what land can or can't be used, but there are pieces of land that appear to be usable, or at least could be altered for use by redirecting water and/or streets. Some are piece of land i've heard discussed for real use, others are areas that i'm just basing on looks. Feel free to correct me if i'm wrong about any of these, I numbered the areas for organization purposes-
Area 1 looks like it could be used, I don't see any major water in the area that would need redirecting. There's the Caribbean Way road blocking access, but that could probably either be redirected or some sort of underpass could be built. Either way it LOOKS like it could be used for a sizable Adventureland expansion if so desired, it's as large as Jungle Cruise and the Treehouse area combined.
Area 2, a cast parking lot. I have heard this being discussed for potential expansion in the past. So I would guess that area is usable (though perhaps inconveniencing the cast somewhat, dunno).
I believe Martin once posted a Magic Kingdom Phase 2 plan with at least part of area 3 being intended for use, I think this is the area he mentioned between the old Skyway bathrooms and Carousel of Progress. Though I also believe back then the canal was smaller and less fat (still I would assume they could probably fill the land and redirect the water somehow). This and area 2 could double the size of Tomorrowland. I assume the expansion pad Martin mentioned was east of the Fantasyland Station is the area I marked as 4, though there is again also a large-ish body of water that may or may not be an issue to deal with. There does appear to be plenty of land farther outside the park, but i'm unsure whether they can dig extra water basics to compensate. You really could basically double the size of Fantasyland if all of this land were usable.
Areas 5 and 6 i'm not sure about. I'm unclear as to whether there's a problem using these areas due to the fireworks fallout, I believe the area where the fireworks are shot off is the pond above Floridian Way. So maybe, maybe not. If so, area 5 is approximately three times the size of Little Mermaid's show building. There was a topic back in 2012 made by whylightbulb regarding a possible blue sky Frontierland expansion being discussed internally, and I believe area 6 was discussed in the topic (the speculation I believe involved altering the riverboat area and TSI and building a walkway on the bank alongside the edge of Haunted Mansion). It appears large enough to fit both Haunted Mansion and Small World combined.
Area 7 I am assuming is the piece of land Martin mentioned usable west of Big Thunder. It's a large chunk of land that doesn't appear to have many necessary alterations to be usable. Assuming all of it could be used, it could provide for a sizable Frontierland expansion (it looks like you could even fit Big Thunder, Splash Mountain and Pirates inside it).