Thats what I'd be voting for
They made too big a deal in Endgame for Cap staying back not to have any consequences. I'd say him staying back maybe "merged" the MCU timeline and the past timeline that they all went to in Endgame. This sets up the multiverse plot for Spiderman, leaves the door open for past Loki who teleported to return, and maybe to eventually drip-feed x-men into the universe
Marvel/Disney obviously do a lot of forward planning, and it all really makes a lot of sense if thats the direction they want to take it
The best thing to do is keep Avengers and X-Men in their own separate universes and have the occasional rift open up to allow a crossover between the two in an Infinity War/Endgame style event. Galactus is the most obvious choice for allowing this to happen. He could meddle with both of them.
Captain Marvel has already set up a moronic situation where we are expected to believe that the most powerful character by far in the MCU is "too busy" to be called upon to neutralise major threats. To merge the Avengers and X-Men worlds would also require a ridiculous explanation as to why Professor Xavier and friends were also too busy to help out the Avengers in the past, or an even more ridiculous retcon of the origins of the X-Men.
The X-Men universe has more than enough characters, many different teams (X-Men, X-Factor, New Mutants, X-Force, Excalibur) and lots of bad guys without having to involve the Avengers for little reason other than fan service. The Avengers universe is also busy enough as it is already with second and third tier characters and plenty more of these boring ass f**kers on the way.
If they start pulling out cheap crossover movies then the MCU is dead. An X-Men and Avengers crossover needs to be at the same magnitude as Infinity War/Endgame. Anything less will be a waste of time..