There's a few possibilities I can think of for the site of a new DVC:
Here's the Grand Floridian site. The spa takes up a relatively small footprint, and there isn't too much space to work with unless they expand into Seven Seas Lagoon, thus blocking the sightline to Cinderella Castle from the Wedding Pavilion. (Any development here would encroach on the Wedding pavilion.) Given that they would have to build a parking lot and support facilities, that really doesn't leave enough room for any sizable DVC resort.
First up is the area west of the Grand Floridian. Property development maps show that this area has the potential to be an extension to Seven Seas Lagoon. A canal would connect the lake and resort sites to Seven Seas Lagoon, and there is enough land for two to three hotels. A few months back, Disney renewed a permit with the Army Corps of Engineers, which permitted Disney to construct an extension to Seven Seas Lagoon within the next ten years, along with a few other projects. Whether or not it's a real possibility or not, I don't know. It would require a ton of earthwork, not to mention clearing many trees and vegetation.
Next is the STOLPort area, which is also designated as a resort expansion site. A canal (between Fort Wilderness and Wilderness Lodge) would connect a new lake to Bay Lake, and there's enough room for one or two hotels. There has been a few land clearings near this area as of late, but the land was most recently cleared in 2000 according to satellite photography.
They could also build a DVC at this site, designated as a possible hotel site. It was at one point to be the site of The Persian, but is now known as the Boneyard, or Pluto Park. I would say it's unlikely they would build here anytime soon.
Here's my personal favorite. The former Venetian/Mediterranean site is designated as a future hotel/expansion site, despite being rumored to have unsuitable ground. Land was cleared here for monorail sightlines before. This is arguably the most valuable piece of hotel real estate at WDW, and there are two theories as to why nothing has been built here yet, one is due to "negative skin friction", a foundation engineering phenomena (thanks to Master Yoda for the information) and the other theory was that the Med was scrapped voluntarily by Disney, as it was supposed to be a twin five-star resort to the Grand Floridian, and seeing that the GF never reached that status, they lost the desire to build here.
Just north of that site is the area of land west of the Contemporary, formerly the site of the rumored "Kingdom Suites".
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As an aside, there is land designated east of the Wide World of Sports for resort development. In this exact area, cleared trees are visible in the May 2010 satellite photographs. I'm not sure why they would want to build here due to its remote location (right across from Pop Century), but this may be the droid we are looking for.