Back when Fantfourstic was in production, I was convinced for a number of reasons that the initial stages of production were a sham on Fox's part to generate leverage in a deal for the rights. Those reasons included that the majority of the cast was not hired until after filming began, the sets were minimal, the script was basic, and filming began at the last minute before the rights would expire, among other things.
I followed the production closely, and felt that Fox was trying to maximize a sale price. Not only that, but Marvel has no real intention to buy when they felt they could get the rights back for free if Fox had no genuine interest to make the film. When Marvel didn't bite on the deal, Fox was forced to finish a production they weren't committed to, cut their losses, and recoop as much money as they could. The horrible and chaotic production just doesn't make any sense if there was no other ulterior motive behind the obviously weak effort.
Even if Fox was only making the film to maintain the rights, part of that effort would have included attempting to investigate the profitability from a sale vs the cost/benefit of a production. So a conversation with Marvel had to happen on some level.
This means the opportunity still exists, and here's where we stand.
Fox wants enhanced rights for the marketing of X-Men toys, television, and other products, and Marvel is in a position where many of the characters associated with the Fantastic Four, could be substantially beneficial to them now that they are beginning to make concrete plans to move past the Infinity saga.
The leverage for any deal is the question. Marvel has enough unused product to live on without this deal, but would certainly want it to happen. Fox certainly realizes they have no value left in trying to market the FF brand and would want some value out of it without having to produce it again. The only way the brand has any value is with Marvel, and so a deal of some sort is sitting right there in plain sight. Whether that includes Marvel giving over a large share of the toy profits for X-Men, or only buying access to certain characters is something to be revealed in the negotiation, BUT!
I am nearly certain that come the end of Infinity, Marvel will announce that they have access to some if not all of the Fantastic Four universe.