Wait I thought Fox owned the X-Men TV rights?
Technically, Fox don't really 'own' anything Marvel, but they apparently have airtight contracts that allows them to keep the live action
movie rights for the X-Men & FF as long as they want, as long as they keep meeting whatever the deadlines are between flicks. It seems they can do whatever they hell they want with the movies too.
For a TV series Marvel cannot do a live action one for X-Men or FF, but Fox can, however they
will need Marvel to agree to it as live action tv rights are not automatically granted by Fox's contracts (contract details do confirm that*), hence the apparent negotiations.
*This detail was perhaps the closest thing we are ever likely to see about these contracts, coming out in the court paper rulings after the Mutant X case ruling. Fox have the live action rights for 'theatrical release movies', but Marvel retained the right for permission to any TV shows to be made.
Fox seem to want the X-Series big time so I would think the Mouse is gonna want a good deal in return. I cannot see it being a rights reversion for the FF, but they could end up getting a good few supporting characters back (like Galactus and the Surfer, The Watcher etc, who, going by the trailer, wouldn't fit in well with Trank's FF anyway).