Well in order to keep Potter the financial juggernaut it is, it makes sense to space out anymore Potter expansions at least a few years. Let Diagon and Hogsmeade continue to print money so to speak while still "new" to most guests. My guess would be we don't see a Potter expansion until 2018 at the earliest with Imo 2019/2020 even more plausible. Also think of it this way if Universal does go ahead with building a true 3rd gate I'm sure the IOA effect taught them a lot. So to ensure that crowds visit USF and IOA in addition to a brand new park I would think opening a major attraction at the other parks would help eliminate a new park siphoning off guests from the old parks.
I mean think about it, if you open a brand new park in like 2020 and nothing else that may negativiely impact the other parks. But if you'd built something major in IOA in 2019 along with opening Potter Phase 3 in 2020 now guests have a real reason to visit all 3 resorts during this trip instead of just the new park. I think we're gonna see something like this but that's just my opinion.