AK is no longer being actively marketed. It's not technically sold out, but there aren't many points left either. GF is selling better now and looks like it could be sold out in a year or so. I am guessing Poly goes on sale sometime next spring for a fall 2015 opening. It will probably not take more than a few years to sell out the Poly points so they would have to break ground pretty soon for the next one if it's complete new construction if it's just converting existing rooms at YC/BC or some other resort.
The EPCOT rumors seem to be the most active right now. The River Country rumor seems to have died down due to the potential cost. Both would be pretty ambitious projects involving some serious infrastructure and major construction. They would need to do demo, build roads, drainage, sewers, parking lots and all of the other bells and whistles needed for a brand new resort. Converting deluxe rooms is much cheaper and quicker. The supply of possible deluxe rooms is running out though so eventually they need to tackle a new resort.