I’m guessing that ET will stay open through construction and year one of the new land... and then I could see Storepants being returned to Pokémon, ET torn down for a new Pokémon flat ride, and a Zelda attraction built towards the back. Possibly even taking a chunk of Garden of Allah out with it?
That would be acceptable to me. I’d say Mario, Legend of Zelda, and Pokémon are “top three” most recognizable brands for Nintendo... while everyone might want a big land for Pokémon and LoZ, I’d have a hard time seeing them flesh those out (without getting too same-y). Pokémon might hem a little too close to Suess Landing as a full-blown land, and I can’t imagine what tie-ins a Hyrule would have with theming and food options to keep it distinct from Potter (or LotR, if that’s on the table).
Personally, as much as a whole Nintendo park would be possible, I think Universal has much bigger fish to fry, and I don’t want to see them spread Nintendo over to IOA or the new third park. Keep it contained in the KidZone area, and keep MiB and the expansion area next door for something else, to keep diversity in the park (fingers crossed for classic Universal monsters!).