I remember seeing an aerial of the Seven Seas Lagoon after the original MK park and resorts were built. And you could see that the land prep for the Asian and Venetian resorts had been done - the squared area on the west side, and the fan-shaped area on the east side. Unfortunately as mentioned the oil crisis and other factors killed the additional resorts.
The Asian Resort land eventually became the Grand Floridian.
The Venetian land has stability problems, having apparently never settled enough to safely build on. In the late 80s they wanted to build a Mediterranean Resort there, but decided that it couldn't be done. There was a report a couple years ago that they tested the land again and still found it unsuitable. I think at the time they were considering the possibility of a true DVC monorail resort there.
They may still have plans for something there in the long term, if it ever becomes possible to build there - it would be prime resort land.