When Disney reacquired the Marvel properties that were controlled by Fox, it paved the way for the House of Mouse to use characters in the MCU that were previously unavailable to them. There’s been much speculation on how the likes of the X-Men, the Fantastic Four and Deadpool will be introduced into the shared universe, then, and in the case of the latter, there’s a suitably demented possibility.
From its inception, events in the MCU were ultimately driven by the power of the six Infinity Stones, but now that they’re destroyed in the present and the duplicates used in Avengers: Endgame have been returned to their original points in time, their place in the ongoing saga is seemingly at an end. However, crossover media and alternate realities have given birth to five other gems (as the comics originally referred to them): Ego, Death, Build, Rhythm and Continuity. And as you’ve likely assumed, the latter of these is the one in question.
It’s appeared only once, in a single issue of Deadpool back in 2014 where the Merc With a Mouth stole it from a sleeping Thanos, and with it he was able to retcon Marvel history. In typical Wade fashion, he planned to use this godlike power to merely get petty vengeance on Siryn, a former subject of his infatuation who abandoned him when his insanity became too much for her to deal with. Of course, Wade has technically already rewritten reality by using Cable’s time travel device to change the past, so stumbling across an object that can alter established continuity is really only taking things one step further.
As ScreenRant explains:
Deadpool’s story with the Continuity stone is wacky in many ways, but in addition to giving the user the power to speak to the comic book writers and editors, the stone also enables its user to change the stories at will. Since one of the biggest questions regarding Deadpool in the MCU is how his story will fit into canon, since the first two movies take place in 20th Century Fox’s X-Men universe, Deadpool could use the stone to write himself into the MCU timeline. No further explanation would be needed, and it’s something fans could easily accept.
Such an occurrence would obviously fit right in with Deadpool’s history of playing fast and loose with plausibility, and also allow him to become a part of the MCU in a fashion that ties in with the movies’ tone of light-hearted lunacy.