Personally I'd do it like this. it's 1961 and the man we know as Kycellius escapes oppression in his home country to the safety of the US. He attends MIT as a grad student and befriends Reed Richards, his girlfriend Sue Storm and her younger brother. Ben Grimm is Reed's Air Force buddy. The man we know as Kycellius' real name is... VICTOR VON DOOM.
Victor wishes to reclaim his homeland, now known as Sokovia and researches the ways used by HYDRA/Red Skull as well as more mystical and scientific means. After Sue chooses Reed over Victor and an ugly situation that results in Ben punching him, Victor flees and disappears.
In 1967, Reed Richards is one of the foremost scientific minds in the World, when Victor reappears, now profoundly wealthy. He claims he has the means to beat the US and Russia in the space race and offers Reed and his team the chance to join. Having concerns over NASA's true plans, Reed agrees and in secret the group launch and successfully land on the moon.
On their return however, they inadvertently stray into the path of an object offering strange red radiation. They return home all changed and as we know The Fantastic Four. Shortly after their return, Victor Von Doom stages a coup in his home country, renaming it to it's traditional name Latveria and installs himself as a dictator. The Fantastic Four repel his scheme to threaten the world realising that Victor had stolen the source of the radiation for himself while Reed hopes to use it to help young adults and teens who appear to have a mutation.
In a climactic battle , the radiation seems to come alive (it is in fact the Aether/Reality Stone) and all 4 disappear,with the timeline altering. It is claimed few remember that time, but they are normally said to be imagining it. Those mutated people also seemed to disappear. This is all narrated to us in flashback by Alicia Masters, an old woman who certainly does remember the Fantastic Four as she loved one of them, even though she never saw him.
Infinity War ends, and in the chaos, that reality begins to warp back into 2016... Those mutants begin to appear again, Sokovia is renamed Latveria by its new leader Lucia Von Bardas with the aid of a mysterious benefactor and Reed Richards and the Fantastic Four, fresh from 1967 arrive at The Baxter Building... only to see it called Avengers Tower!
For me, this would rock as it means we'd get to see Mads Mikkelsen as Doctor Doom, which is perfect casting... it's maddening he was wasted on Kycellius, but if reality is warping, he might have gone a different way and become Doctor Doom instead.