One of the biggest surprise cameos in Guardians of the Galaxy: Vol. 2 came from the race known as the Watchers, who are most commonly associated with the Fantastic Four property. Kevin Feige explained how Marvel Studios were able to pull this off in spite of Twentieth Century Fox owning the Fantastic Four film rights.
Some overexcited fans may have seen the Watchers making a cameo as a sign that the Fantastic Four property is finally back home with Marvel Studios after Fox’s unsuccessful reboot more or less killed the franchise. However, Kevin Feige clarified in an interview with /Film that while the Fantastic Four film rights are still at Fox, there is a grey area which allowed them to use the Watchers:
“There’s joint custody with a number of things. There are certain characters that they have, but races that we share.”
So it appears as though Marvel Studios is free to use the Watchers as a race, but they may not be able to use specific Watcher characters like Uatu due to complications with the rights – a situation which also applies to the Skrulls, even though they have yet to actually appear in any MCU movie (or Fox movie, for that matter). It’s worth noting that Fox actually had the rights to use Ego the Living Planet while Guardians of the Galaxy: Vol. 2 was being written, but they signed them over to Marvel in exchange for more creative freedom for Negasonic Teenage Warhead’s set of superpowers – so it might be possible that the Watchers appearing in a Guardians movie at all were a result of that deal.
There are no Fantastic Four movies in development at Twentieth Century Fox, as the company seems to have given up on making movies with that license and are instead making the most out of their X-Men movies. In the meantime, Guardians of the Galaxy: Vol. 2 is playing in theaters right now. The film stars Chris Pratt as Peter Quill (Star-Lord), Zoe Saldana as Gamora, Dave Bautista as Drax, Vin Diesel as Groot, Bradley Cooper as Rocket, Michael Rooker as Yondu, Karen Gillan as Nebula, Sean Gunn as Kraglin, Pom Klementieff as Mantis, Elizabeth Debicki as Ayesha, and Kurt Russell as Ego. Many of these same characters are set to appear in Avengers: Infinity War, its sequel, and Guardians of the Galaxy: Vol. 3.