Although Avengers: Endgame represents the end of an era, it does mark the beginning of a concept that is central in the comics from which the movies were birthed: The multiverse. Major spoilers for Endgame follow, so if you haven't seen the movie, proceed at your own risk. The film follows the original six Avengers, with the addition of War Machine, Nebula, Ant-Man, and Rocket Racoon, as they travel through time to collect the Infinity Stones. Although I have been very vocal about plotholes, directors Joe and Anthony Russo have cleared the air on Captain America's ending.
At the conclusion of the film, after both Iron-Man and Black Widow die, Captain America decides to go back in time and live out his days with his lover Peggy. The original issue with this ending was the plausibility of it in connection with the movie's established rules for time travel. The Hulk explains that in time travel, you cannot go back and change things to manipulate the future. He mentions that if you go back in time, your future is now your past and you have created an alternate timeline. This is the multiverse theory. The Russo Brothers admitted as much while speaking with the LA Times.
"In the movie, the Hulk is very explicit about what our rules are, which is you cannot change the present by altering the past," stated Anthony Russo. "All you can do by going to the past — and for a character like Cap[tain America], living in the past — is create an alternate future. So this is a world in which alternate timelines exist."
“'Back to the Future' is one of our favorite films, and the rules of that movie are ubiquitous; they have informed pop culture for 40 years," he continued. "And we wanted to do something different. We didn't realize how complicated it would be to create new rules and have people go with us, but we learned very early on in the test screenings that people are really committed to those 'Back to the Future' rules — if you shoot yourself, you should die. We spoke with a few physicists, and there’s a lot of theories about how time travel could work. We chose the multiverse theory."
Now that the MCU has introduced the multiverse, the possibilities are endless. It also creates an entry point for time traveling villains like Kang, and may also serve as a way to introduce the Fantastic Four or the X-Men.