^The thing for me is, I think the Fantastic Four work best as a team that's already extremely well-established -- not just famous in the sense of being a flavor-of-the-month novelty, but in the sense of being establishment figures, the royal family of the superhero community, the anchor of the superscience community. And they couldn't be that if they were added to the MCU, because then they'd be brand new. Reed couldn't become the go-to inventor and supergenius without displacing Tony Stark from that role. So it wouldn't be the same. It wouldn't capture that sense of history and legacy that worked so well in the FF comics I've read (mainly just the recent stuff by McDuffie and Hickman). So I fear it's already too late to incorporate the best possible version of the FF into the MCU. They would've had to be there from the start.
I assume RDJ won't play the role forever, in fact they are already building up a new team and Downey Jr. is really starting to show his age (not that means anything but it looks odd being surrounded by 20 and early 30 somethings). I think he is locked for Infinity War but so far no stand alone Iron Man movies have been planned so he'll be gone sooner or later (and i doubt Marvel will try to find a replacement).
So some years later there could be a place for the FF, maybe in phase 4 or 5 if they make it this far and (and this is the biggest issue) if Marvel gets back the rights fully or wants them bad enough to strike a Spiderman like deal with the other studio.
However they will only have the one shot of saving the movie version of the FF because at least the fans will remember the bad FF movies and would need something extraordinary to go back to the superfamily.
All this hangs on an extremely thin thread and the way the MCU is set up they don't really need them. Would be nice especialy with their cosmic scale enemies like Galactus who would make a good supervillain after Thanos gets defeated.