I have mixed feelings about Fantastic Four.
They're such a product of their times. I think a F4 movie set in the 60s would be incredible, and there's so much fun you could have with the aesthetic. Plus the MCU hasn't really explored that time period.
I've actually spent way too much time thinking about this and have even outlined a script. See, the thing with the F4 is they're ok as a team, but what you really need is a proper Dr. Doom as a foil to Reed Richards, which played right, is played as a pompous tool and Richards' intellectual arrogance drives everything for better or worse.
It works like this - the movie begins with a montage of the F4's origin story done in classic Jack Kirby style visuals, and snippets of life after getting powers - defeating the Mole Man, Sue as the face of the women's lib movement, Benjamin J. Grimm the ever lovin' blue-eyed thing as a black panther, Johnny filming a beach movie, more clips of the F4 defeating their rogues gallery, maybe a team up with Ant-Man since we know he was active then, etc. And interspersed in all of this are news clippings of incredible things happening in Latveria - major advances in medicine and technology, the population thriving, things that imply the influence of Victor Von Doom.
Here's the A plot - in the movie itself, the F4 have things figured out. They're thriving as heroes and individuals in their respective fields of interest. Reed Richards starts to catch wind of the advancements being made by Doom, and is initially a little threatened, but comes around to the idea that they should be allies and put their minds together. Particularly on the one problem that he's never been able to solve - how to turn the Thing back into a normal person.
So Doom is invited to the US on a diplomatic mission. The US isn't sure what to make of him. We discover that Latveria is thriving, but Doom's rise to prominence is a little unsettling, and while benevolent, it appears that he's functioning as a dictator. The entourage of Doom bots he brings as a protective security force is also a little unsettling. Still, he is clearly not evil, and he offers to share some of the medicine and technology he's developed in exchange for cooperation with the US as a world power.
Reed and the F4 meet Doom as part of the diplomatic mission and for a scientific meeting of the minds. Doom casually passes the Thing a gift - some pills he created for him in case he ever wants to become human again. They all laugh at the absurdity of Doom proposing a solution to the problem Reed has never solved but Doom simply offers a friendly smile. Thinking nothing of it, the Thing pops the pills into his mouth.
While touring the Baxter building with Doom the Thing comments that his stomach is feeling a little upset (what was in those pills anyway, doc?) and excuses himself. He doesn't get far and passes out, and his body begins to shudder. Little by little, he sheds the rocks that have made up his exterior until he's revealed to be his normal human self again. Needless to say, everyone is shocked, and Reed is baffled. How did Doom succeed where he had failed, and with a fraction of the information Reed had on hand?
The team dynamic begins to crumble. The Thing is thrilled to be back to himself, but disturbed that Reed is infatuated with Doom and appears to not even care that the Thing is himself again. Sue is disgusted with Reed's obsession, and Johnny is concerned about the optics of the Fantastic Four now being a Fantastic Three, which also puts the Thing further on the outs.
Amid this, the F4 encounter a threat unlike any they have before - the Salem Seven (which tie in very nicely to Agatha Harkness, and are a deep pull from the F4 archives). Their powers suddenly seem useless and inadequate in the face of magic, and the Thing experiences the terror of realizing that without his powers he's unable to protect his team. Doom returns and defeats the Salem Seven singlehandedly, using magic reminiscent of what we've seen from Dr. Strange, as Reed realizes that he hasn't been losing to Doom on science, but due to Doom's unprecedented blend of scientific progress aided by magic.
The Thing asks Doom if he can be made back into the Thing, and Doom gives him another set of pills to turn him back, which he had always created as a counterpart to the original pills, just in case. Reed is baffled by the ease of this and unsettled by Doom even more now that he knows magic is likely involved.
Fresh off the battle with the Salem Seven, the F4 and Doom get news that an anomaly in space has been detected sending a message that appears to threaten the earth, and requests a meeting with their world's most powerful beings. The F4 are asked by the government if they're willing to explore as they represent humanity's best chance to understand the anomaly and are representative of the most powerful. Hesitant, they accept when Doom courageously volunteers.
The team is frazzled, and not sure about Doom, especially given the relationship he has with magic and the unknown, but its off to space they go, where they meet the alien entity Galactus, devourer of worlds. Galactus explains that he has scoured the universe for life fit for his consumption, and has settled on earth as the next planet under evaluation. Galactus explains that earth is not yet "ripe" and he will revisit earth at a later time to partake. The F4 try to argue with Galactus while Doom passively observes, and Galactus leaves the solar system with his decree made.
Meanwhile, a handful of people including JFK overhead the conversation with Galactus having been tapped into the shuttle's communications system. Realizing that the world will panic, and that the earth currently has no ability to defend itself from a threat of this nature, they agree to keep it quiet, and share that in their hopes of creating another Captain America, a new theory has been advanced. Why turn a living man into a superior man, when you can make the man great from birth through genetics? Combining the powers of their scientists with the minds of Richards and Doom, a discovery is made to mutate genes in the human population (you see where this is going).
Realizing that war for the existence of earth is coming, the F4, Doom, and the US government make the decision that knowledge of Galactus' future return is too dangerous for the world. But how can they trust that the small circle of people that know will keep it quiet. Doom proposes a spell we've already encountered in the MCU, and that Doom, the F4, all of the scientists on the mutation project, and anyone else connected to the space mission, all have a spell cast to be forgotten (explaining why no record exists of the F4 and Doom in the world's history), while the F4 and Doom work on their own to explore other means to acquire power on earth to combat Galactus while mutation takes hold and a new generation of heroes can emerge powerful enough to combat Galactus.
The F4 and Doom part ways - the F4 to explore the quantum zone and what secrets it may provide to protect the earth, and Doom to explore the negative zone. In the course of their exploration, the F4's work is interrupted, and they are suddenly teleported to an empty planet where they see a single figure off in the distance. It's Doom, but in a form they don't recognize. Suddenly, other people appear on the planet. They are all on Battleword, setting up...
Secret Wars.
