(Spider-man, Amazing Spider-man, Marvel Cinematic Universe, Possibly Live Action Versions of the Animated Shows)
First on the list is a spiderverse movie. With the various versions of Spiderman over the years, everyone has their favorites. Be it from the old cartoons, Tobey Maguire, Andrew Garfield, or even the one from that odd Japanese show.
How cool would it be to see them all on screen at once? Especially if they throw in some never before seen characters like Miles Morales, Spider Gwen, Miguel O'Hara, or even Spider-Ham.
As cool as it would be to see the avengers clash with the justice league, the x-men universe seems to have much more in common with man of steel then the avengers ever will. From the public's and government's stance on powered individuals, to the more grounded tone, these two universes seem made to connect.
So it would be pretty damn awesome if someday they did. Hopefully before Hugh Jackman ages out of the role of wolverine.
(X-Men/Marvel Cinematic Universe)
This one is much weirder than the previous two, but hear me out. What if deadpool just popped up in the MCU with no explanation? He can complain about not being able to swear, make references to the X-Men universe, and comment on different actor's salaries and contracts.
Then he can go back to the X-Men universe and complain about the superhero registration act, the avengers, and infinity stones. And if successful he can continue to go back and forth with knowledge of both universes without any explanation because he is deadpool.
4. Secret Warriors/Legends of Tomorrow
(Marvel Cinematic Universe/Arrowverse)
With the Secret Warriors coming soon, and Legends of Tomorrow hitting next year, it seems like the perfect time for a crossover. If the Legends chased a villain into the MCU, the Warriors could be tasked with tracking them down. And watching two teams of powered individuals fight is even better when both teams are fully fleshed out and you are used to rooting for both of them separately.
Plus it could lead to some other cool moments, like Arrow bumping into the defenders, flash meeting quicksilver, Coulson and Sara discussing their resurrections....