If in a present day film I'd have them introduced as adults, but yet to become the FF. Doom would be around, but in the shadows about to launch his takeover of Latveria*.
With CM in in the 90's, and dealing with Skrulls, a reference to the theories of a young genius concerning FTL travel would be neat.
*My pitch for bringing the FF into the MCU would be that Reed has been around all this time, but consumed in his research into FTL space travel. Ie: He has had a working theory since college, but no means to make a ship capable of using it.
That changed with Avengers and after the tesseract was lost he got Shield backing to make an FTL drive ship (Earths government seeing the value in having something capable of taking a fight to the enemy, rather than waiting for an Alien invasion to turn up on our doorstep), but that got pulled after Winter Soldier and the collapse of Shield.
Reed was however able to make enough headway to pursue the project, albeit slower, with finances from his own resources/inventions, and post Infinity War he has a working prototype ready to test out (short test runs with animals aboard check out fine). Not being a secret government venture the maiden voyage makes all the headlines and that draws Doom's attention, who harbouring his long standing grudge with Reed, sabotages the ship.
On the test flight it goes wrong, with the ship threatening to explode due to Doom's tampering, but Reed quickly spots the problem and corrects it. However they have been taken off course and on emerging from FTL at the wrong destination they get exposed to exotic cosmic radiation and the ship is damaged travelling through unknown space littered with asteroids (later to be revealed as passing through the remnants of a planet Galactus has consumed).
With Reeds smarts and Ben's piloting skills they manage plot a course back to a Earth for a messy landing, and their powers manifest soon after emerging from the crash. While all that has been going on grabbing the world news attention, Doom, wanting to settle all old scores at once, launches his brutal takeover of Latveria and reveals himself to the world as it's new ruler.
Shenanigans ensue...
Or something like that
(Even that doesn't feel right)...ugh
) seems pretty quiet for what should be a pretty big, enthusiastic production.
