I know people don't like lists, but I can't help myself!
Universal's Fantastic Worlds: A new level of interactivity - you won't interact with the worlds, it will interact with you. The park would be a hub-spoke model leading to the entrances of worlds that would expand into other lands.
Hub: The hub would tie into the reason you can access all the "worlds" in the park. It would be themed like a "Fantastic World" with bursting colors and plants/characters you wouldn't see on Earth. There would be a Main Street walkway to the middle of the hub which would explain the backstory and history of the land.
Original World: Walking straight from the hub would take you to an original world, themed around a space-station. There would be no IP tie-ins, and would feature an indoor coaster and a Spiderman/TF type dark ride.
SNW: Walking Southwest from the hub would lead you to SNW - Mario with MK, Yoshi, and a bullet bill flat. Through warp pipes, you would enter SNW - Zelda, DK, and a mishmash world with Splatoon, Kirby, and Meet and Greets with different characters.
World of Horor - The entrance to this land would be Frankenstein’s castle with a dark ride inside. There would also be separate areas dedicated to Dracula and the Mummy.
Dreamworks World - the entrance would be the Dreamworks Theatre on steroids. Multiple screens, seats that really make you feel the movement, smells. Any step that can be taken to improve the experience would be taken. The separate lands would be Berk, Far-Far Away, and Trolls (which would be smaller than the rest).
Jurassic World - I would like to see a recreation of the park seen in the movies, but that’s probably a bit too ambitious. There would be an on-site hotel overlooking the area, and hopefully a smaller-scale recreation of Jurassic World with a Mosasourous Screen attraction and either the gyrosphere ride or a ride where you have to escape from a new genetically engineered dinosaur.
Wizarding World - I’m no expert on Harry Potter, but Fantastic Beasts is obviously what to do, and I’d like to see the Ministry of Magic shooter connected somehow.
Something that irks me about theme parks is that I can walk into a building a suddenly be transported into a seemingly never-ending land (Forbidden Journey, Simpsons come to mind). Personally, I want to feel like the place I’m exploring is defined and makes sense. This would be very hard to achieve, but any effort is appreciated. Also, major characters would be around for M&G, but minor characters would actually walk around and interact with people, like how it’s done at DHS and USH. The first idea I had was Goombas in SNW and they would be controlled like the trash can anamatronic at Disney a few years ago. Also, I’d like to see some costume characters retired. Seeing a big, fluffy Mario with a permanent grin on his face doesn’t really entice me to go into a line to meet him. In their place would be anamatronic a that could move their torsos and arms, and can blink/move their mouths with pre-recorded phrases. This is also ambitious, but I think it could be done with characters like Mario. Everything in the worlds should be natural and not feel forced or out of place.