Maybe do a version like the Endgame Avengers time traveler suits?
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Actually though, one of my concerns is Where TF was the FF during End Game? Or were all 4 of them dusted?
I hope they don't lean on an origin story to hard. I think that would be a mistake.
The tricky thing might be to avoid doing an Avengers style fight movie and I think fighting Dr. Doom would unfortunately be that. I think the Fantastic Four needs to be the Science counterpart to the Dr. Strange.
Maybe deal with the Negative Zone or the fallout from the Infinite Stones destruction.
Or they help setup Secret Wars.