No Fox, No Fantastic Four?
A regular topic of conversation on the Screen Rant podcast, it might sound counterintuitive but there is a downside to Marvel Studios owning the film rights to all of their characters - since certain heroes and villains are more bankable than others. Without question, Marvel Studios (via Disney) is a major powerhouse in Hollywood these days - with a complicated, multi-media, shared universe plan that stretches beyond 2020. However, even though the studio is signing actors to ten-plus picture contracts, Marvel still has limited resources. The highly profitable studio doesn't have a blank check to produce a new film every month and, even if they did, they'd still be at the mercy of limited industry assets (sound stages, crew members, visual effects companies, etc) that are required to actually take a movie from script form to the big screen.
Additionally, should Marvel manage to outbid all other competitors and secure enough real estate and personnel to actually churn out a steady stream of MCU films, the studio still runs the risk of cannibalizing their own individual films and sub-brands at the box office, at home, and in merchandising. Given that Marvel is ramping up production, aiming for two or three films a year, they've yet to hit the point of over-saturation but that doesn't mean they can't overproduce.
If you accept Marvel is limited in how many movies they can make each year, then it's not a stretch to see that once certain characters start filling in available slots, even over multiple years, others will be left out in the cold. As a result, should the rights for every Marvel property suddenly land back in Disney's full control, then the studio has to start deciding which characters get a movie, which ones simply cameo in other films, and which ones are sitting on the sidelines entirely. Fantastic Four hasn't proven to be a sure-thing franchise for Fox (much less a particularly hot commodity on comic store shelves in recent years); meaning, even if Marvel Studios acquires the rights, Disney may never green light a new film based on the characters.
Conversely, it's equally possible that if characters like the Fantastic Four and the X-Men ever made it back to Marvel, then the studio would be less inclined to take risks with oddball properties like Guardians of the Galaxy. One year later, we know that Guardians of the Galaxy paid-off (and was a refreshing change of pace in the genre) but it wasn't always a risk-free hit - and there's no way of knowing, for sure, if Marvel Studios would have ever proceeded with the film if they had well-known galactic-traveling characters like the Fantastic Four available to them. Essentially, if a Fantastic Four movie ever gets a green light from Marvel Studios it will mean that some other character (or hero team) was passed-up in order to bring Reed, Sue, Johnny, and Ben back to the big screen.
For that reason, there's a significant (though often dismissed) benefit to non-Marvel studios producing films based on Marvel properties - since those films don't take away from other ideas the home team might choose to produce. Since Marvel hasn't been busy with X-Men: Apocalypse, Deadpool, and Fantastic Four, they've been able to start developing new movies and TV series (based on characters that audiences have never seen before): Doctor Strange, Black Panther, Captain Marvel, Inhumans, Jessica Jones, and Luke Cage.
That all said, between Story and Trank's Fantastic Four movies, coupled with news that Fox intends to build an entire shared universe around the X-Men, to include solo stories like Deadpool, as well as X-Force, among others spin-offs, Fantastic Four could still be better off at Marvel - even if Marvel has no plan to use the titular foursome in a solo movie any time soon.