Endgame sounds too much like the MCU under Kevin Feige is going to be making a drastic change after phase 4 ends.
Sure the x-men universe joining the MCU will change things (like use of additional characters and villains).
But what mega-storyline does the marvel comics have that use all those characters together (other than the X-Men vs. Avengers)?
As others have said, this will be the last film for most of the Avengers for contractual reasons. So most of the main six (Cap, Iron Man, Hawkeye, Thor, Hulk) won't be seen again. They signed up for many, many pictures when they were far more unknown, and command much higher salaries now: even Marvel couldn't have made
Infinity War if they were paying the three Chris-es their current average salary.
And after
Avengers and
Avengers: Infinity War the big "crossover movie" feels done. I don't think Marvel can do that again. They'll likely focus on their other side characters (Captain Marvel, Ant Man, Black Panther, Doctor Strange) and possibly the Eternal And then whomever they can get from the FF and X-Men.
And maybe build to another Avengers movie later, possibly with more side characters. You can imagine a Captain Marvel led Avengers with Beast from the X-Men, Valkyrie from Thor, Sersi from the Eternals, Crystal from the Inhumans, and the newly added Black Knight or Quasar.
These Avengers can have their own story and adventures, but different in scope.