'Guardians of the Galaxy Vol 3' could establish the X-Men's place in MCU by introducing Shi'ar and the Starjammers
Speculations are running wild as to which comic book storyline will be adapted for the upcoming 'Guardians of the Galaxy Volume 3' with suggestions ranging from 2006's 'Annihilation' to the more recent 'Infinity' storyline from 2013. Of course, James Gunn isn't one for adapting stories panel for panel from the comics so it's just as likely that what we're going to get might be something completely fresh.
Now that Thanos (Josh Brolin) is dead, there is bound to be a power vacuum in the Galaxy, which multiple warring races will no doubt attempt to fill. This could open the door for the Shi'ar to make their MCU debut as invaders from another galaxy and where there's Shi'ar, the Starjammers are never too far behind.
The Shi'ar are a race of bird-like aliens from the Andromeda Galaxy who have been longtime enemies and sometimes allies of the X-Men. They command a vast empire in the comics that rivals the Kree and the Skrulls, not to mention their technology is far superior to that of most other species. Unfortunately, they are also prone to genocide and slavery so they aren't exactly the nicest aliens out there.
The Starjammers are a group of space pirates with a special vendetta against the Shi'ar Empire. Their membership has changed a few times over the years but the founding members of the team include the green giant Ch'od, the cat-like alien known as Hepzibah, the formidable warrior Raza Longknife and their fearless leader Corsair, better known as Christopher Summers, the father of the X-Man Cyclops/Scott Summers.
While the Starjammers may not be mutants themselves, bringing them in would give the X-Men an easy way into the Marvel Cinematic Universe without having to go too deeply into their backstory. Even if no actual X-Men members were to make an appearance, their existence in the MCU could be established just by having Corsair talk about his son and what he has been up to.
This would be an excellent way of establishing the existence of the X-Men within the MCU without diverting too much attention from the actual plot of the movie. The reason why the Starjammers would work so well in a 'Guardians' movie is because the space pirates are essentially a more serious, respected group that would serve as a great contrast to the wacky, fun-loving Guardians.
There's plenty of comedic potential in the characters and they would fit in perfectly with the sort of tone we have come to expect from James Gunn's 'Guardians' films. In fact, it would be absolutely amazing if the Starjammers were brought in not as allies but as enemies who are trying to hunt down the renegade Gamora (Zoe Saldana) for the greater good of the universe, only to have them put aside their differences to unite against a common threat. The jokes pretty much write themselves.
Whatever Gunn has planned for audiences, we won't be seeing the movie for a while as the director is currently busy working on a new 'Suicide Squad' movie for DC. 'Guardians of the Galaxy Volume 3' does not officially have a release date yet but it should be making its way to theaters in early 2022.