The 'lost in the 60's or 70's' thing could play into that though. Have Reed be a colleague of Pym's or a student of Stark Sr., involved in much of the background setting. The incudent happens and the FF are created and arrive in the present day...
No, that wouldn't be what I want, because they're still
new to people in the present. Having their powers would still be a novelty and they'd need time to establish a new status quo, to get used to being in the present. That's exactly the opposite of what I'd like to see, which is a story
after they've already built up a years-long legacy as the anchor of the superhero community. I don't want their powers to be new. I don't want them to be just starting out as superheroes. That's not the end of the story that's interesting to me. That's the end of the story that's failed to work cinematically on three occasions now. I don't want to see another iteration of how they get their powers. Their powers aren't even the interesting part of the Fantastic Four. What's interesting is the institution they've become. Their real power is the power of reputation and fame and influence and trust earned over a long history of achievement.
And I just don't see any way to reconcile that with the MCU, because they don't already exist in the MCU, so they'd have to be brand-new if they were added. Tony Stark already occupies the niche that Reed Richards would need to fill. That history and legacy would not be there.