In the mid-'80s, Marvel wanted a reason to group all their biggest characters together into one story…so they could make toys out of it. No, seriously. That's why Secret Wars, one of the comic book industry's first and biggest cross-title events, even exists.
In the story, a mysterious cosmic entity known as the Beyonder has been observing the Earth and all its mighty superheroes for years. One day he decides to cosmically kidnap them all and transport them to Battleworld, a planet of his own creation. His plan: to force the heroes to duke it out. The winner will, essentially, be granted anything they so desire.
Secret Wars was never considered a particularly great story, but its premise is perfect for a MCU adaption. Imagine, in addition to post-credit scenes, small cameo appearances where the Beyonder is hidden away as an Easter egg in the background as he observes the events of each and every Marvel movie (and show) after Infinity War. He can be the Marvel version of the Observer from Fringe. He's always there, always watching. By this point, all the major players in the MCU will have long since been established. They're all regularly showing up in each other's movies and are all deeply involved in each other's lives. After various philosophical differences between them all raise tensions to their peak, they're all whisked away to Battleworld to settle their scores once and for all.
But what if the Beyonder has always been there, lurking in the background of EVERY Marvel movie? How could this make sense? According to a fun fan theory, Stan Lee, with his many cameo appearances, is the Beyonder, and one day he will transport all of his creations to another planet to watch them fight. It'll never happen, but if it did, it'd be one of the best twists ever.