Bear with me because this is quite left field speculation.
If we think of the point of view of a Disney exec they want to maximize guest spending at the most popular park on the planet. So they likely want to go big with this expansion. The new expansion would have multiple paid lightning lane attractions, shops, food locations, meet and greets etc.
If it’s a mystery IP that doesn’t fit well in Magic Kingdom, but could potentially be quite the draw and check boxes for a massive land then it’s probably not the traditional Disney or Pixar animation franchises. Which would make me think Lucasfilm or Marvel or even Fox owned franchises would be there.
Except that Star Wars and Marvel have had quite a few studios misses recently and already have their own dedicated areas. So what property could fit? Lord of the Rings.
But Disney doesn’t own Lord of the Rings! Ah yes my dear reader they do not. But Warner Bros is desperate for cash and has not been doing well. Disney also announced a partnership with Warner Bros with respect to streaming rights this week. With the new lord of the rings movies down the pipeline and past Disney efforts to acquire the property, this could make sense. The possibilities of a lord of the rings land would be endless and could possibly give Universal quite the run for its money.
I know wild speculation but maybe it’s a bold move to reinvigorate the Walt Disney company with yet another merger or at least partnership and this is one of the pieces of the puzzle. I would still reserve this for a dark horse speculation however.