The year is 2012, and pretty much everyone in the world has just gotten their minds blown by Marvel's The Avengers. As audiences walked out of the theaters, they all began to wonder when they were going to see Spider-Man or Wolverine show up in the next Avengers movie. This was really the first time the public began to understand just how complicated a custody agreement with Marvel and their characters could be.
From the 1970s to the mid-1990s, Marvel Entertainment Group sold the rights of various characters, since they weren't yet in the business of making their own movies. One of these deals was sending the X-Men, Daredevil, and the Fantastic Four over to 20th Century Fox. Spider-Man headed to Sony, Blade went to New Line Cinema, Ghost Rider to Columbia, Punisher to Lionsgate, and the Hulk went to Universal.
However, Marvel soon began slowly re-securing some of these rights, and by late 2013, the Punisher, Daredevil, Hulk, Blade, and Ghost Rider were all back at Marvel Studios. (Note: Universal still has some of the rights of Hulk, which is why we won't see a solo movie anytime soon). Sony finally released their death grip on Spider-Man ever so slightly in 2015, when they worked out a "shared custody" agreement with the Marvel. 20th Century Fox, however, has kept their hands on the X-Men and Fantastic Four.
The thing is, when Fox bought the X-Men and Fantastic Four IPs, they got a lot more than just that. Fox currently owns not only the X-Men, but the entire concept of being a mutant — even if they aren't traditionally an X-Men. Meanwhile, owning the Fantastic Four means a lot of the more cosmic people of the Marvel Universe, like the Silver Surfer or Galactus, are also in Fox's hands. This made things complicated — but it was worse than we thought, as we discovered several years later.
It appears that there are some characters that fall into a "grey area" where both companies hold the rights. The most prominent examples are Quicksilver and Scarlet Witch. While technically the children of Magneto are mutants, they also fall under the Avengers banner, so both companies hold the rights. That's why we got two versions of Quicksilver. And rumor has it that this was the reason Quicksiver was killed off in Avengers: Age of Ultron — to kind of split the difference. Fox gets to keep Quicksilver while Marvel keeps Scarlet Witch, despite them being siblings and their relationship being a very important part of both of those characters developments.
To back up a little bit, one of the main reasons that Fox made a Fantastic Four movie two years ago is because they were about to lose the rights to the characters. The studios have to keep the lights on in these various departments or Marvel can ask for their toys back. This is essentially what ended up happening with Daredevil; they went too long without using the character.
While Fantastic Four is certain the most egregious example in terms of quality, Fox also made X-Men: First Class for the same reason, and it turned out fantastic. It's the same reason why Sony rebooted Spider-Man. It's moviemaking as dictated by contract lawyers and not creatives, which is why it so rarely works out.
That horrible Fantastic Four movie led a lot of people to think Fox would just sell the rights back to Marvel. The franchise and name were basically dead in the water, and a third reboot just seems like a terrible idea. There were a lot of rumors that got even louder when James Gunn had to arrange a switch with 20th Century Fox to gain the rights to use Ego the Living Planet.
Kurt Russell['s Ego, the Living Planet] in the new Guardians movie was the character that Fox swapped with Marvel to [change] Negasonic Teenage Warhead powers. – via The Playlist
Things have been quiet for the last several months, with Fox even saying that they aren't completely ruling out a sequel to Fantastic Four. In a brief sequence and in the final after-credits scene in Guardians of the Galaxy Vol. 2, we got a look at characters that were once thought to be owned by Fox, which led to speculation running rampant all over again. This is minor spoilers for the movie that doesn't reveal any plot points and is really more of an Easter egg:
In one of the scenes we get a look at the Watchers. The Watchers are an ancient alien race who decided it was their duty to help the lesser evolved members of the galaxy. However, when their first attempt resulted in tragedy, they vowed to take a more anthropological stance and never interact with other races again — hence the name "Watchers". They are characters that first appeared in the Fantastic Four, so everyone always assumed that they belonged to Fox. When they appeared in Guardians of the Galaxy Vol. 2, a lot of people thought that Marvel had gotten the rights to the first family of comics back. In an interview with Kevin Feige over at /Film, they asked him directly about this:
And that's also a question I had, the Watchers, aren't they Fantastic Four owned by Fox? Has there been a deal made?
No. There's joint custody with a number of things. There are certain characters that they have, but races that we share. If that makes sense?
So apparently that joint custody agreement has more than just the Maximoff twins in it. It apparently also includes the Watchers, and judging by the way Feige worded it, perhaps there are more in there as well. Namor, the Sub-Mariner, is often considered part of the Fantastic Four group and is referred to as a mutant; but he debuted in his own World War II-era book. So perhaps he's under that umbrella, and we might be able to see Namor's grumpy face in the MCU.
There's still no word as to whether or not Marvel is trying to get the rights back to Fantastic Four, or if Fox is even opening to selling — but what was once a blood feud seems like it might be toning down. Marvel and Fox worked together on the fantastic Legion, and that worked out great for all involved. If the deal for Spider-Man ends up working in Sony's favor, then perhaps the doors will be open for more cooperation.
Or maybe both of the studios will look up someday and see the dump truck of money they could make if they did an Avengers vs. X-Men movie.