A 2nd water park I'm a bit skeptical on. UOR isn't very fond of how problematic VB has been operationally. Maybe they change their mind if this VB special ticketed event proves popular. (I could see a scenario where UOR builds a water park inside of a hotel though).
But yes, continued buildout with more hotels should keep Disney on their toes.
I know Disneyhead said 3 more hotels, but you can fit a bit more. (Green is what UOR owns and can build on - they own more through Shingle Creek, but doubt they touch that anytime soon).
You can fit another Terra Luna-sized hotel complex to the west of Terra Luna. There's a bit of land between Stella Nove & Epic Universe that can fit a hotel the size of Aventura (doubt they build a tower here, could see an expansion of Stella Nova with a parking garage later on.)
To the west, UOR owns a massive piece of land that can easily fit in Endless Summer Dockside with more room to spare for an additional Aventura-style tower and enough space left over to leave some greenery.
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This is just speculation, no credible rumor on here. There's a lot of room to play around here, especially if you consider part of the Orange County owned land by the Convention Center (across the Catchlight Crossings development) that UOR can purchase from the county for additional hotels or even a parking structure.
I still think UOR should "cap" their resort at 3 parks until they fully flesh out IOA, USF and UEU.