Retro 60s Sci-Fi aesthetic (it would take place in the 60s)
Emphasis on space exploration, somewhat Rick and Morty style. Let them visit some weird and unique planets
The group gets hit by cosmic rays during a trip to space. The plot centers around them gathering resources from different worlds to help them get back to Earth. Emphasis on Reed being resourceful and smart as fuck and knowing how to survive the dangers of space despite it being his first time in space. Nonetheless, they either get stuck in space until modern-day MCU or they stay in space until then because there's some larger cosmic villain they need to battle or hold back. Whatever happens to them prevents them from aging.
The villain would be Psycho-Man, a denizen from the Microverse who controls a robotic human-sized body and has the ability to manipulate emotions. He'd be able to take advantage of the fact that the group is in a stressful situation with powers they don't fully understand. He'd be another pawn of Kang, who's using him to manipulate events to get....whatever desired outcome he's after I guess lol.
Doom would only be alluded to, maybe a quick cameo prior to him becoming Dr. Doom. The implication would be that Doom, much like The Ancient One, was able to use magic to prolong his age. I think Doom's backstory, much like Kang's, should be given to us in bits and pieces throughout different MCU media. They should be set up as rival villain factions fighting for control of the multiverse leading up to Kang Dynasty and Secret Wars.
Reed should be early-to-mid 40s, Ben early-40s and Sue can be late 30s, Johnny can be mid-to-late 20s.
Foreshadow Secret Wars and have Reed see a quick peek at his evil ultimate universe self The Maker or something. I like the idea of a council of Reeds, and this being something that influences his behavior leading up to the next Avengers movies.