I could only see them building on the site of the long proposed water park near DL (which was that very expensive strawberry field on Harbor Blvd, near the convention centre). This is not next to DL, but the only area significantly large enough which would allow for a building which is somewhat offset from traffic, etc.
Remember, even the Grand Californian was built on premium space and doesn't have the extensive grounds of a Wildnerness Lodge, etc
I would love to see a California
DVC at a lower point structure than the current DL/GC structure, but I think that unless Disney owns a good chunk of land somewhere else around DL, or has no more plans for a water park and/or third park, it would be tough to squeeze it in.
I was thinking about the old campground next to the DL Hotel, but I can't recall if something else is there now, like the new parking structure. I also recall being told that Disney has been buying up land over the past years in Anaheim (such as the former Grand Hotel lot)