Fantasy is a unifying theme because fantasy describes something. It's not just a word, it describes what's in the land. Each ride feels like a fantastical journey. In tomorrowland, each ride should have a futuristic vibe. In frontierland, each ride should have a frontierish vibe. Sometimes that falters a bit and that's what leads to weaker lands.
Pixar is just a word. Perhaps I went too far before, so I must directly ask you, what says that Zootopia can't be part of a Pixar Land? What says that Big Hero 6 can't be part of it? Just a name? I'd like to do a test. Sit 100 people in front a screen and have them watch Big Hero 6, but without the words Disney or Pixar appearing at all. After the film, would they be able to tell which studio created it? If they can't tell, well then, why couldn't the movie fit in a Pixar Land?
Star Wars is their main movie that embodies the sci-fi/adventure genre, which makes it disntictly different from most of what Disney Animation and Pixar put out. Marvel is their main cluster of movies which embody super heroes and such. You can usually tell just by watching the movie if it's a superhero movie or not, although sometimes it gets close with plain sci-fi. People have argued that Big Hero 6 and GotG could work together in a superhero land, and I can't argue that. Onward... In a way, ToT represents the horror genre. Rock 'n' Rollercoaster is the odd child but it has a distinct rock n roll, music, gritty feel. A land of family movies would work in contrast to everything else in DHS.
And so yes, I'd be a supporter of a land that embodies family movies (ya know, movies that make you all warm and fuzzy inside). That is, if they can create a setting that a) doesn't just feel like a bunch of places thrown in a blender and b) works in contrast with the rest of the park. But Wreck It Ralph, Bolt, Chicken Little, Big Hero 6, Zootopia, Planes*, etc. would all fit such a land. They could certainly choose not to use any movies outside of Pixar. But if they're looking for a place to put a Zootopia ride, I wouldn't want them to avoid putting it in the land simply because it's made by the wrong studio.
Lastly, I must wonder, will Pixar even exist in 5 or 10 years? The next CEO could just combine it with WDAS. If you think I'm crazy, let me mention that our friend Spirit sees it as a strong possibility too.
*Okay, please, no Planes. On a side note, I wonder what studio most people would say that movie is created by.