How does Marvel not have all the leverage here? If we assume a negotiation is taking place, From an audience standpoint Fox is negotiating from a position of weakness with a FF property that they themselves tarnished. Moreover, from a financial standpoint, Fox stands to lose $60 million dollars on the film, with nothing to suggest that they can transform this into a moneymaker (especially when you factor in that outside of the first Story film, two of the films performed miserably in the box office). Marvel's going to say, "why should I have to offer you shiny new assets for a property that is damaged and has no money potential?" Fox has no strength that they can play on with this negotiation.
Marvel does not have all the leverage because 20th Century Fox is not in the same position that Sony Pictures was in. With Sony Pictures, Spider-Man was essentially the only viable franchise for Sony and they totally botched it up and they had the loads of bad press regarding the e-mail hacks.
Even though this particular iteration of Fantastic Four was a bomb, 20th Century Fox still has other IPs they can lean on (X-Men, Planet of the Apes, Ice Age, Alien/Predator/Prometheus, etc.) and they can find new ways to use the Fantastic Four like introduce a new version in an upcoming X-Men movie or just simply make another one. Unlike Sony Pictures, which could not afford further tarnishing the Spider-Man brand, 20th Century Fox can afford to make another dud of a Fantastic Four film.
Also, look at the position that Marvel Studios is in. Is there really any room for Fantastic Four with their film slate? They're busy focusing on their established franchises on top of starting new ones like Captain Marvel, Doctor Strange, a new Spider-Man, etc. Also, Marvel Studios wants Fantastic Four, but they don't need it.
And finally, 20th Century Fox doesn't need an X-Men television series. They want it, but they don't need it. Their television production arm is rather lucrative thanks to Modern Family, American Horror Story, the Simpsons, Family Guy, X-Files, etc.
While Marvel is in a good negotiating position, it really has nothing to do with the failure of Fantastic Four, and if they're too demanding in return for giving up the X-Men TV rights, 20th Century Fox can just walk away and not care.
Do you think that's a realistic scenario? Pressure from shareholders and investors is going to force them to explain why Fox should keep the IP rights when it has proven to perform terribly. On top of that, shareholders and investors are going to be very reluctant to spend any more money on the FF as they haven't gotten good returns on their money, and with this movie, they've lost a good chunk of change. A sequel is definitely out of the question considering that the actors and actresses involved appeared to have a miserable time, and the movie itself is viewed upon so terribly amongst audiences that it's a waste of time and money to revisit it. So that leaves a remake as a possible option, but do you really think shareholders want to spend money on a remake of a property that doesn't exactly suggest that it has money-making potential? That being said, it seems like Fox has more of a soul-searching journey to do, and less likely to hold onto the Fantastic Four property onto its cold dead hands.
Shareholders and investors aren't the ones who greenlight films. While I certainly agree that we aren't getting a sequel to Trank's Fantastic Four for the obvious reasons, but Fantastic Four is still an IP with potential. And imagine how angry shareholders and investors would be if 20th Century Fox gave up the rights to Disney/Marvel only for them to make an extremely successful Fantastic Four film.
Now don't get me wrong, I would love an MCU Fantastic Four film and if the Fantastic Four rights are a part of a swap involving the X-Men TV rights, I would be ecstatic. But I think that the discussions are going to be more focused on the financials and technical details of the X-Men TV series (advertising, digital and home video distribution, the channel it will air on, etc.) as opposed to any rights reversions back to Marvel. And if there are any rights swaps, I think it'll be the swapping of rights that are useless to Marvel Entertainment and 20th Century Fox. Merchandising rights are useless to 20th Century Fox the same way the TV rights are useless to Marvel. Character rights to more cosmic based characters like the Shi'ar and Skrulls are useless to 20th Century Fox based on the direction they have taken with the X-Men IP while they can be used far more usefully in Marvel IPs like Guardians of the Galaxy and Captain Marvel.
For me, it's really about not getting my hopes up. I have no desire to be disappointed in the likely event that the rights do not revert in these negotiations. Any TV deal I'm expecting these aspects:
1. Marvel Entertainment gets a cut of the revenues from advertising and home video/digital distribution.
2. The X-Men television show will air on either ABC or FOX.
3. Marvel Entertainment gets 100% of merchandising rights from X-Men and Fantastic Four, the same way they got the Spider-Man merchandising rights back from Sony Pictures in return for giving up their cut of the box office. The Walt Disney Company's forte is in merchandising, and the biggest reason why we don't see X-Men or Fantastic Four toys anymore when the movies are coming out is not because the movie is being made by a competitor, but because they would have to share it with 20th Century Fox. Notice how Disney did not care about selling Amazing Spider-Man 2 toys when they had 100% of the merchandising rights to it (even though Marvel Studios hated that movie). And 20th Century Fox will probably be very willing to give those rights up because the Walt Disney Company has made the merchandising rights useless for them.
Other things I see as more likely than them giving up the Fantastic Four rights, not expecting, but see as more likely:
4. Marvel Entertainment either gives up their cut in the X-Men movies in return for something or renegotiates to have a slightly higher percentage of the X-Men box office.
5. A reversion of rights to minor characters that 20th Century Fox does not need but Marvel Studios can use quite effectively. Think more along the lines of Galactus, Silver Surfer, and other characters I have previously mentioned.
6. An X-Men TV series would be a co-production between 20th Century Fox Television and ABC Studios.
EDIT: 7. 20th Century Fox Home Entertainment gives up home video and theatrical distribution rights to the Star Wars films and television projects they have distribution rights early to the Walt Disney Company's subsidiaries.