The earliest that we will see a new park in WDW would be no earlier than about 2018. We will see one eventualy, but before that we will see some major refurbs like the SSE, HM, and PotC. Disney realized this after AK's opening. There is no need to build a new park now, when Disney built AK they thought that people were going to add an extra day to their trip to accomadate 4 parks rather than 3. Another thought that they had behind it was that more people would come. Neither of these have turned out as they were expected to, instead, people merge in AK & MGM into single days and the overall attendance did not rise. MK, EPCOT, & MGM are still recovering to their annual attendance numbers before the opening of AK. So, based on that: there will need to be an overflow of people in the park (enough around the year that Disney has to do something), and the existing parks must be up to date and are full day parks so people do not have as much of an option to cram two parks into a day. The earliest that I see this happening under these reasonings is 2025-2030.