I just don't see that happening. Fox isn't going to just hand it over after a couple of years. Something needs to be worked out. This wheeling and dealing for certain characters doesn't fly for me. For one, Marvel is asking for way too much, and if they made that offer to me as an owner of FF, I would have laughed at them.
But Marvel has a reputation of low balling so I am not surprised. They are going to have lower their asking price if they want FF and/or related characters. If I am Fox, I would gladly let DD revert than take anything close to a deal like that.
To be fair, we really don't know what's on the table either fully character wise, rights wise or fanancially. All we've got to go on are at best 3rd/4th hand reports and clear signs of both playing negotiating tactics. We don't know the full extent of Marvel's offer (if there is one) and we can only guess what FOX want out of it, if anything.
A lot of fans are interpreting both how they want in multiple ways. We've been discussing everything from a loan of SS/Galactus rights for an extention of DD rights to Marvel gunning for the entirety of Fantastic Four to revert to the MCU in exchange for continual rights to DD and everything in between. Some of those might be considered lowballing others might be a fair exchange.