Practically immediately following the closing of the Disney and Fox merger, Walt Disney Studios started using stills from the Deadpool franchise in their marketing. Fast forward a few months and Disney then uses Ryan Reynolds' fan-favorite character in a sizzle reel of Disney-owned properties at the biennial D23 Expo. With all of the promotion, it'd seem the Merc with a Mouth would be joining Marvel Studios' Marvel Cinematic Universe sooner rather than later, right? Well, not according to one popular new fan theory.
In a short but sweet theory shared online by u/Joonicks, it's technically impossible for the two different worlds to co-exist, at least in their current setups. As the theorist points out, Deadpool 2 featured the element of time travel thanks to the introduction of Cable (Josh Brolin). In the Deadpool sequel, Cable traveled with a teddy bear that was changed in the present by events that'd happen in the past.
As it was explicitly stated in Avengers: Endgame, that's not the set of time-traveling rules the MCU follows. Instead, the current science behind the MCU's time travel means each time someone goes back in time, they simply create a new timeline instead of displacing themselves in the current timeline. So long as they return things to the norm in that newly-created timeline, it will simply merge back into the main time stream with new ill-effect whatsoever.
In fact, the Ancient One (Tilda Swinton) explains the to Hulk if they do manage to change massive events, the timeline will end up doomed for all eternity, even though the Sorcerer Supreme didn't care to elaborate on that.
Either way, Marvel Studios head Kevin Feige has admitted they're more than likely going to keep the Deadpool franchise in the hard-R demographic, so it has yet to be seen how they plan on introducing him to MCU.
Upcoming Marvel Studios projects include Black Widow on May 1, 2020, The Falcon and The Winter Soldier in Fall 2020, The Eternals on November 6, 2020, Shang-Chi and the Legend of the Ten Rings on February 12, 2021, WandaVision in Spring 2021, Loki in Spring 2021, Doctor Strange in the Multiverse of Madness on May 7, 2021, What If…? in Summer 2021, Hawkeye in Fall 2021, and Thor: Love and Thunder on November 5, 2021, and Black Panther 2 on May 6, 2022. Marvel Studios Disney+ series without release dates include Ms. Marvel, Moon Knight, and She-Hulk.