Until they put up construction walls, it's a rumor. It's been gone over before, but MSHI ain't going anywhere. Disney isn't going to pony up $100M+ to make it go away until they have concrete plans of their own for WDW. With Avatar and Star Wars on the way, we're looking at 2020 before they even start thinking about Marvel in WDW.
That said, Universal had the Hulk movie license, which Marvel wanted back. Marvel also wants The Fantastic Four to fail so they can fold them into their ridiculous Universe. So the RUMOR is that Disney went to Uni, hat in hand, to get Hulk and to get rid of Dr Doom. RUMOR also has it that Disney handed over Hulk info, and permission to IOA to upgrade that coaster. But what else will MSHI get for closing one of their rides? It's gotta be something good. Not "creating a new Avengers ride" good, as has been making the rounds. Disney isn't going to approve any E-Ticket in their biggest rival's park. (Personally, I'd give them the theme park rights to the Marvel Netflix shows.) Time will tell...
Now that we know MSHI is set for the future, the four biggest problem areas in Universal Orlando are Toon Lagoon, Lost Continent, San Francisco and KidZone. Toon Lagoon would be the easiest to convert to Nintendo, but it would be the most difficult to clone over to USH. USH doesn't need a new flume and there's just not enough space for a rapids ride.
I said elsewhere that retheming Lost Continent to Zelda would be perfect, but Zelda would be a harder sell than Mario, Luigi, Yoshi, Donkey Kong, etc. And Pokemon too. That's a land you create after the original Nintendo Land is a huge hit. And, really, there's no reason they can't pull a Harry Potter and have Nintendo in both parks... And a third one!
San Francisco would be a distinct possibility, IF it wasn't slated for Fast & Furious. The disaster building is almost perfect for that.
Which leaves KidZone, the smallest, scariest land. The temptation would be to create a big, new ride - say, Mario Kart - and fill in the rest with flat rides and/or play areas, a la Springfield in USF or Super Silly Fun Land here. Unless they dump Animal Actors (unlikely) and/or ET, there just isn't going to be a lot of room for ride buildings. And we most certainly will not get an outdoor Mario Kart here. Our only hope is either ET going away or KidZone taking over a portion of the empty lot next to The Simpsons with actual ride buildings and not outside, screamy rides.
I have a feeling most of the rumors for that land will be similar to the rumors for this one, so we all need to keep an ear open for those.