Cable's time-traveling device in Deadpool 2 may be the key in bringing the X-Men into the MCU. Since news broke out that were purchasing Fox's TV and movie assets, Marvel fans have been theorizing how the mutants and their related characters will be tied into Marvel Studios' existing universe. Over the last several months, countless claims have come out, but the latest one from the pile suggests that Josh Brolin's second comic book character - Nathan Summers - holds the key to making this happen.
Unlike the X-Men and Fantastic Four film series, things are a bit more complicated when it comes to the Deadpool franchise integrating into the MCU. While Kevin Feige and his team have yet to give away any inkling on how they're incorporating the long list of comic book heroes previously with Fox, the mutants and Marvel's first family are almost going to get a reboot to better fit in the MCU's established lore. Ryan Reynolds's film series, on the other hand, is confirmed to be carried over.
Given this, people are convinced that the Deadpool franchise will be the entry point for the X-Men to the MCU, including Reddit user xinfinitimortum, who posits that Cable's (who is originally from around 50 years in the future) time traveling device could be the key to all of this. According to the theory, the wrist device that Cable used to jump through time in Deadpool 2 may be the same device used by the Avengers for their Time Heist in Avengers: Endgame. Created by Tony Stark to function like a time GPS, the gadget helped the heroes land in their targeted periods and prevent them from getting lost in the Quantum Realm. Cable may have found one of those buried under the rubble of the old Avengers HQ, enhanced it via the technology from his time, and used it for himself.
Other than the fact that both Cable and the Avengers' time traveling device are both worn on the wrist, there are a couple of other points that support this theory. When Cable first showed up after he made the time jump from the future, he was wearing a white ensemble. Although quite different from the Quantum Suits that Earth's Mightiest Heroes wear in Avengers: Endgame, it could be a variation of that. Perhaps it's tweaked in a way that it no longer needs the Quantum Tunnel to operate. As we saw in Deadpool 2, Cable simply used his gadget in the middle of the rubble Russel left after Firefist killed his family.
At this point, it's anybody's guess how the X-Men will be brought into the MCU. But Avengers: Endgame left several ways to do that including this coincidental opportunity which seems like a solid path in making it happen. Linking the existing lore to at least the Deadpool's franchise via the wrist device effectively gives them the opening they need to slowly incorporate what's essentially separate universes at this point. Even if Deadpool 3 moves forward still with an R-rating, it can function like a spin-off branch of the MCU. But in the event that Marvel Studios want to bring the Merc-With-A-Mouth, Cable and their tie-in characters into the Avengers' turf, they have an option to execute that without making things very complicated.