Marvel want all their rights back. The reason why they'd particularly want the F4 rights is they gave away loads of associated rights with them, which means they can't use whole swathes of their established canon. They can't use 2 of their biggest villains, they can't use a whole race of aliens that have been in loads of plotlines, etc etc. The actual F4 isn't what will interest most people, though Marvel will take some pride in getting their 'first family' back and they'll use them. They don't add much to the current roster as it stands, but having Dr Doom back does. And post Infinity War they might be a bigger deal. FF are an odd one for me. They're seen as somewhat of a 'red headed step child' by many Marvel fanatics but i've always been a fan of them. I certainly think there's the scope for a decent movie in there and I had high hopes (given the director and the cast) for the latest incarnation but my lord was it dismal.