Fox wants to create an X-MEN TV Series, but they need Marvel's Permission, now regardless of what you think about the X-MEN or whether or not they fit in the MCU lets take it off the plate and lets take into the consideration that Fox needs Marvel's permission to make the TV series.
I have my doubts that Marvel will let Fox make the TV Series, and the Fantastic Four aren't a way of letting that happen as they are now worthless brand.
1. If Marvel trade the X-MEN TV rights for the Fantastic Four there is no guarantee it will be as successful, and if they give Fox the rights which will make spme competiton for them when they could get the rights for a price or for free, and not having Marvel properties competing against itself.
2. Marvel doesn't want any other properties that belong to them in the comics in any universe, they want all their properties back or at least in the MCU(See Spider-Man),if Fox has their way it will be in the new Earth-10005, but Marvel doesn't want their properties competing against them or at least their universe.
3. The Fantastic Four the only thing that could have possibly gotten Marvel to say yes to a FOX X-MEN TV Series is now worthless if the Fantastic Four movie was somewhat good, and it didn't bomb at the box office, and the comics sold well then Fox would have more bargaining grounds to trade for X-MEN TV series approval.
So unless the X-MEN TV Series is in the MCU I don't see why Marvel will trade for the Fantastic Four, or will give it to Fox freely(yeah right)