Avengers 4 will be a game-changing chapter of the Marvel Cinematic Universe saga, one that Marvel Studios executives have already teased will re-shuffle the characters of the MCU in big ways. Ever since the Disney / Fox deal went through, fans have been looking at Avengers 4 as a much bigger opportunity to bring Fox's X-Men and Fantastic Four character into the MCU.
As you can see above, one fan on Reddit has created a post-credits scene for Avengers 4 that teases the introduction of The X-Men and Fantastic Four franchises into the MCU, while managing to add a nice little Deadpool cameo for good measure.
The method and timing of the X-Men and F4 joining the MCU is still something that is still being furiously debated by the fans, and is probably a logistical knot being untangled by Marvel and Fox right now. We've heard that the transition could take over a year, which could make it hard to make such a shift in Avengers 4.
Of course, the second half of the MCU's "Infinity Saga" could leave the door propped open for Phase 4 to tell a story of mutants being revealed.
Avengers: Infinity War opens in theaters on May 4, 2018. Thor: Ragnarok is now playing in theaters. Other upcoming Marvel Cinematic Universe movies include Black Panther on February 16, 2018, Ant-Man and the Wasp on July 6, 2018, Captain Marvel on March 8, 2019, the fourth Avengers movie on May 3, 2019, the sequel to Spider-Man: Homecoming on July 5, 2019, and Guardians of the Galaxy Vol. 3 in 2020.
The X-Men film universe continues with The New Mutants on April 13th; Untitled Deadpool Sequel on June 1st; and X-Men: Dark Phoenix on November 2nd.