I stand by my belief that KH4 won't have "conventional" Disney worlds. It doesn't make sense to have something like Moana in the VR verse when it would just as well be in the main KHverse. KH4 is the perfect time to do SW/Marvel/Fox stuff and get it over with.
Muppets Treasure Island is better than Treasure Planet anyway
Switch can run UE4 and it never got KH3 (natively). That said, one of the big rumors from last year's Gamescom was Switch 2 supposedly running The Matrix Awakens demo, which isn't just UE5 but looking pretty darn darn more than anything on the market.
Given the absence of Nintendo at this year's Gamescom, they may have pulled out because of that leak, giving weight to this possibility.