It is a known fact of both cinematic features of the Fantastic Four being critically critiqued by fans, and meeting horrible outcomes in both domestic and international box offices. With that being said, Fox is still insistent on continuing their attempt at bringing the Fantastic Four to the big screen despite their failures in ownership of the property. The failures of the Fantastic Four films aren't due to the title of the property not being popular among fans, there's a die hard following for the Fantastic Four, the failure of the films are due to studios like Fox withholding the rights to their characters to themselves. Withholding character rights isn't the reason that the Fantastic Four films have failed, but in isolating the Fantastic Four off in Fox's single universe, FOX studio has set themselves up to fail by isolating a team of characters who have built their history in the jointly combined Marvel universe.
The Fantastic Four are an integral part of the Marvel universe, and have played integral roles in several of Marvel's more historic story lines. But to have the team of the Fantastic Four adapted to a cinematic feature where they are isolated in a world in which other integral Marvel characters should exist alongside the Fantastic Four but their existence is not even acknowledged; trying to tell any story around the Fantastic Four would be difficult to do. That is why both attempts at bringing the Fantastic Four to the big screen were so roughly fleshed out. In any film, the story does have to center on either a specific character or group of characters but centering a film on a group of characters like the Fantastic Four who rely on a larger ensemble of characters to tell their story. Not including any of those secondary, supporting characters to reinforce the main characters' existence in the story makes the Fantastic Four existing in any real world setting seem implausible to the audience.
The Fantastic Four haven't been given their fair due in a cinematic telling yet but there's still opportunity for the Fantastic Four to get a justified story despite the property rights belonging to Fox. Fox needs to be listening. Sony just met some unexpected disappointment with the lukewarm reception of the Amazing Spider-Man 2 which pushed Sony to cancel production on three possible Spider-Man centric films. Things started to look dim for the future of Spider-Man until an agreement was made between Disney and Sony to allow Spider-Man to cross over into the central Marvel Cinematic Universe. Sony and Disney property rights of Marvel characters can now be shared between films. The agreement between the studios has reinvigorated fan interest in Spider-man, and the character of Spider-man also gets the opportunity to be presented in a loyal adaptation in comparison to the two previous iterations of Spider-man on the big screen. Fox can learn from Sony by allowing the Marvel titles they own the rights to, to cross over into the MCU which could reignite fan interest in the Fantastic Four and also provide Fox with necessary profit from Marvel titles that have been bleeding money out of them.
The Marvel Cinematic Universe is crowded with characters as is, and is only getting bigger in scale with more films being produced, and the addition of the Fantastic Four wouldn't be the ideal choice for Marvel right now but the characters of the Fantastic Four still could have a place in the MCU. Currently, the MCU is centering around the Avengers and the next Marvel film, Captain America: Civil War which will most likely see a fully formed team of Avengers by the end of the film. Although not every Avenger will make it on the main roster of the Avengers, so that leaves multiple Avengers without a place to go. There could be a team of Secret Avengers being formed but it's more likely that an 'A' and 'B' team of Avengers will be formed in lieu of Secret Avengers. Were a B-team of Avengers to be assembled, there would surely be a place for one or more members of the Fantastic Four to compete for a place on the Avengers, but that's neither here nor there. To sum up the point, Fox can benefit from allowing their failing Marvel properties to cross over with Disney and Sony's Marvel properties; crossing over properties would reinvigorate fans' interest in forgotten properties and also make up for the losses incurred on the production for all three Fantastic Four films. And as of right now, the MCU is too overstuffed with superheroes to reintroduce the Fantastic Four to the ensemble of the current MCU. There is a chance that the Fantastic Four could find a legitimate place in the MCU in the future but for now the Fantastic Four needs to stay put.