It would most likely be DisneySea. As far as i know it was discussed back when Tokyo DisneySea was still in the planning stages. However, back then AK was still too "fresh" and it was decided (rumored) to concentrate on the existing theme parks instead. I'm surprised this is even a topic now as DHS (and Epcot) did not get the necessary attention. Building a new park now would be bad timing.
First priority should be (and people say this for 15 years now)
1. Master plan for extended Monorail system with increased traffic flow. (including a connection to all parks and the rumored new park) and an interconnected nighttime service running between hotels and the nightlife entertainment areas.
2. Replace team in creative control over Epcot with a more experimental mix of younger people who are closer to today's technology. (It's EPCOT, not Oldcot (and neither New Fantasyland)) They might think Frozen will raise Epcot's overall annual attendance, but a proper done redesign of attractions and infrastructure, with sticking to the original Epcot concept, would raise overall attendance by more than a third over the next decade. Frozen will only bring a short term success and it will raise attendance only by 5-9%
3. Pick up a $650,000,000 budget for Epcot and update the park to a more modern standard, plus a general redesign of the entrance. But not without staying true to its original concept (Epcot taught me a lot when i was a kid, it's outright unfair to the younger generations to get more dumbed down "edutainment" with each passing year)
4. More "simple" leisure entertainment and free recreational areas around the Resort area.
5. Make sure to keep all 4 park concepts unique towards each other (Frozen in Epcot... will just transfer Epcot into a second MK.. Magic Kingdom Mark II
)
6. Once attendance numbers stabilize again, start construction on park no. 5
(the site should be about halfway between MK and Epcot)
So my idea, my guess, is DisneySea.
