Last month, Disney bought large chunks of 21st Century Fox's assets, and provided the deal is approved, it will mean big things for the Mouse House, including being able to include the X-Men in the Marvel Cinematic Universe. Fox acquired the film rights to the mutant characters in 1994, and because of that, with rare exceptions (Scarlet Witch and Quicksilver among the most notable), the MCU has never been able to include characters from that corner of Marvel Comics. However, assuming Marvel Studios had been able to launch its shared universe with both the X-Men and fellow Fox property Fantastic Four at its disposal, Guardians of the Galaxy director James Gunn suspects that his movie starring the group of lovable a-holes might not have happened. As he put it:
For a long time, many Marvel fans have hoped that the X-Men and Fantastic Four would be incorporated into the MCU, and thanks to the Disney and Fox deal, it's now actually possible. However, as James Gunn answered on Twitter when asked if Guardians of the Galaxy would have been made if Marvel had the X-Men and Fantastic Four from the start, his 2014 movie might have been set aside due to a packed slate of movies. On the one hand, Marvel Studios head Kevin Feige was interested in making Guardians of the Galaxy, so that's a point in the movie's favor. On the other hand, if there were X-Men and Fantastic Four movies being released alongside the likes of Iron Man, Captain America and Thor stories, then Gunn is likely correct that there wouldn't have been enough room for Guardians of the Galaxy. Which isn't to say that we couldn't have still seen the Guardians lead their own theatrical tale, but it probably wouldn't have happened for a long time.
If you're like me, when the Guardians of the Galaxy movie was announced in 2012, you raised an eyebrow. Even for longtime comic book fans, the Guardians were relatively obscure characters, so giving them their own movie was a strange idea. Thankfully for Marvel, that strange idea paid off and then some. Guardians of the Galaxy was a bonafide success upon release in 2014, making over $773 million worldwide and turning its starring characters into household names (well, Groot at least). Three years later, Guardians of the Galaxy Vol. 2 followed and also impressed critically and commercially. So yes, it's taken a long time for the prospect of the X-Men and Fantastic Four appearing in the MCU to finally be within grasp, but because we got these versions of the Guardians in the meantime, I'm fine with the tradeoff.