Will a "Secret War" bring the Fantastic Four and Doctor Doom to the MCU?

[Flash Gordon]

I'd like to see the Fantastic Four as NASA astronauts that were lost in space/time decades ago, and suddenly reappear in modern day space. That would keep their space race sense of urgency and explain where they've been without having to provide an origin. Maybe the Silver Surfer plays a part in this. Perhaps Galactus is coming.

The X-Men can just be introduced as something happening now. Maybe mutants have been something of urban legend, but now with social media and the internet it is getting harder and harder to hide. Xaiver offers a sanctuary for those who can't seem to get by in society due to mutations/gifts.

No need for any big multiversal crisis, just introduce these characters into the ongoing narrative of the world that has been created thus far.

[SuperiorIronman]

In-universe, Thanos is what sparks the idea to get to space and study the cosmic forces at play because Earth feels it needs to be better defended against a threat like Thanos in the future. It's not that the Chitauri didn't do that before, but it's a much bigger thing than the Avengers can handle so the world is now dealing with it "hands on". Hence you get The Fantastic Four charting into the unknown and getting hit with the cosmic rays (perhaps a byproduct of using the Infinity stones?). With all this energy in one place it also attracts Galactus who will try to consume Earth and the vast quantities of energy left over from the Infinity Gauntlet. Perhaps a good reason Thanos was the first being to use the stones all at once, it attracts a cosmic Grim reaper (whether intentional or not it attracts a sort of death in Galactus) potentially explaining why the stones are hidden. Thus in a Doctor Strange movie charting aspects of the multiverse they end up briefly in the MCU Negative Zone setting up Annihilus. Finally Doctor Doom has complete precedent considering the events of the MCU, and how important it is to horde technology and artifacts for self-preservation of his country. Also it establishes a potential threat to Doom. Real world counterpart to the intervention in other nations by the west and the in-universe threat of super beings especially deriving power from space. Latveria may not be as prominent because Victor hadn't come to power yet or simply hadn't began hording the technology and artifacts.

No 60's FF for them or there villains, you can do it simply by building off of existing continuity.