Here's the thing about the rights expiring in 2022...when I look at what's working with the MCU right now and try to map out Phase 4 films, there really isn't any room for a Fantastic Four film until 2022 anyway, and even then it's a tight fit unless you bump the MCU up to four films a year. I'm not sure Disney would ever do that, because it really starts to crowd other properties (like Star Wars for example). Anything is possible, of course, but the price for that sort of decision would be at least another billion dollars of revenue per year from MCU films.
You could also introduce the team in a different film (Spider-Man/Civil War style) but even then, what film would that be? Looking ahead, the earliest choice that would make sense to me would be Black Panther 2, which could come out in 2021.
When I look at it that way, I can sorta see why Marvel would just call Fox's bluff and wait around, especially now when they've got so much going on and there's a 0% chance of a sequel to Trank's thing. If they want to do something with Black Panther 2, they still have until about 2020 to get a deal made and, the fact is, the longer they wait the greater there leverage is in negotiations.
I think there's room for 4 Marvel films ( if you add up the Fox, Sony and Marvel films, we're currently getting more than 4 films a year already). It used to be that all the big films came out in Summer, but we've seen that schedule stretched to the point we can get big films pretty much any time... but we still have a relatively small number of big films coming out.
This is probably pushing it a bit, but I could imagine Marvel doing a new film nearly every month if they wanted to. Imagine going to a Marvel film and knowing you'd have another one coming next month. Comic books worked on a monthly schedule.
But while I think they could do a film per month, the real issue is content and that's where the FF come in.
Marvel could have easily sat back and said: "You know, people like Iron Man, Captain America and Thor. Let's just release sequels of those films every year and everybody will be happy."
But obviously they went a different route than that and mixed things up and brought in new characters and that has worked very well for them. And while Spider-Man and X-Men were getting stale, Marvel was producing different films with different flavors featuring different characters.
But what new characters do they have for the coming years? Black Panther and Captain Marvel will keep things interesting for the foreseeable future, but then what? Sure, they can pull out relatively minor characters (like Gaurdians of the Galaxy were prior to their film), but that's not as much of a sure-thing as having well-known characters to fall back on. They got a little lucky (management might say they chose wisely) with James Gunn and GOTG, but a lesser director could have easily gone sideways with a property that didn't have a built-in audience.
And people want to see the big-hitters. Iron Man and Captain America were good films, but people also wanted to see Iron Man and Captain America.
There aren't any big Marvel characters I can think of that haven't been exploited yet, so the FF would be a big injection (not only the FF, but related characters) of big, fresh, lucrative characters for Marvel to use down the road.
