• this originally was just about Mutants but after my post I realized it could also explain Fantastic Four, and once I got to writing about them it also became about the true nature of the Bifrost and The Infinity stones. I added it all in a edit marked bellow

So this is a pretty quick and simple theory. It has been seen that when exposed to cosmic energy from the infinity stones, ones genes can be mutated causing them to get special powers, examples are captain marvel, scarlet which, and quick silver, these could potentially be the first three confirmed mutants.
In endgame rocket raccoon says that an unprecedented amount of cosmic radiation/energy was released from the Infinity when thanos snapped that engulfed the planet, This could be used as a catalyst to start a boom of new mutants and people gaining special powers.

Now this leaves the question what about older mutants such as wolverine, professor and magneto? Well it’s wouldn’t be the first time that people came into contact with cosmic energy, during captain America the first avenger, hydra used weapons powered by the tesseract, meaning allot of soldiers on both sides got exposed to cosmic energy, and this can be used as a retcon for the second wave of mutants, and the first wave would have came when Odin first hid the cosmic cube on earth, which could be used to explain the really old mutants.

But now one might wonder why mutants would only Now be coming out to the spotlight light now? There is actually a quit simple explanation for that, being that there were so few mutants they tended to hide there gifts from the public fearing discrimination, and those who dis reveal there gift were often assumed to be god because they were so rare, but now that there will be a boom of mutants , there will be so many mutants that the can no longer remain hiding in the shadows.

Edit: This can also be used to introduce Fantastic Four , who in the comics get there powers after being exposed to cosmic rays while on a experimental space flight. You can even keep the part of them getting hit by cosmic energy while in space. There are three ways to do this.

The first and potential easiest being that after the snap all space travel and/or exploration except that conducted by the avengers. A scientist named reed Richards, having seen aliens first hand is inspired to build a rocket and travel to outer-space, he designs an experimental ship, that he Johnny Ben and she use on an unauthorized mission to space during the events of endgame and are in space and get hit by cosmic energy released from infinity stones when hulk snaps everyone back to earth or when tony snaps his fingers to get rid of Thanos (personally I think tony sent him back to 2014 and wiped his memory, as to not create any holes in time) thus giving them powers.

As for the second and third potential ways for them to be introduced both of them would mean they get their powers before the original snap.

The second potential time for the Mishna to gain there powers would be during the invasion of New York. When Loki opened up the wormhole during the invasion of New York in the first Avengers film, he uses the tesseract/space stone, which shoots a beam of cosmic energy towards the sky in order to open the worm hole. There ship is either getting launched at the time or already in space and gets hit by the cosmic energy released by the space stone. As for where from then through the event of endgame, either all of that energy palaces them all into a coma or they got sucked into a worm hole and transported to the future. In real life based on real scenic the most theoretically plausible form of time travel would be by traveling through worm holes, while it isn’t the time stone the science of wormholes is what validates this possibility.

The third possibility for when/how they got their powers, is based on the Thor: The Dark World Prelude comic, in which Heimdall uses the the tesseract to restore the rainbow bridge allowing for the use of the Bifrost once more. According the entry about the Bifrost on its entry on fandom.com linked below, the Bifrost is a form of inter dimensional energy allowing for instantaneous travel with in the Nine realms, and the rainbow bridge is the Asgardians means of harnessing the energy to access the Bifrost in order to travel the nine realms. What if the Bifrost was the name given by the Asgardians to the pure raw cosmic space energy that allowed for transportation to any where in space not just the nine realms —Hella implies this when she says she and Odin could have conquered the entire universe if they wanted but Odin had a change of heart, implying that the Bifrost could truly allow travel to anywhere but the Asgardians limit there travel to the 9 realms;— and the space stone could potentially be just a single conduit of this cosmic space energy/force allowing anyone to tap into its pure form(with each of the other infinity stones being conduit of its own respective force, as the Collector explained in guardians of the galaxy before the universe came into existence there were six singularities, and out of these six singularities the infinity stone were formed, in this theory those singularities are pure cosmic forces and the stones are conduits made out of a pure form of the energy of these force that allow there user to access them , and that the rainbow bridge was a less pure conduit that was dependent on energy from the space stone to power it, and that’s why it could only be restored by the space stone, and it was used by Asgardians to channel the cosmic space force which was known to them as the Bifrost . Furthermore it is even possible that the tesseract was a housing unit built by the Asgardians to siphon of energy from a the space and transport this energy through space to the rainbow bridge, which then used that energy both as a power source and to search the cosmos for its source with which it matches the energy of space stone to so the Asgardian can then access this force and manipulate it to travel where ever they want.

As for how Heimdall transported hulk to earth using the Bifrost, He simply used dark magic (second link Heimdalls fandom.com MCU wiki entry) which is know to be a way to access the bifrost(first link ). As for how Thor accesses Bifrost with storm breaker, it was created to counter act the infinity gauntlet and be able to withstand blasts created by the infinity stones, meaning it could potentially be designed to harness the cosmic force just as the infinity gauntlet, but due to it not being forged out of the pure energy of these forces as the infinity stones were it doesn’t have the same level of reality warping power. This is further supported by the fact that it is enchanted to Eitri to take advantage of several weaknesses of the infinity gauntlet, and what is the biggest weakness of the gauntlet? That it requires the infinity stones to harness the cosmic forces. Meaning the perfect counter to that is something that wouldn’t require the infinity stone. Or you can just say he also enchanted it with dark magic so it could access the space force/Bifrost

As for how this relates to the fantastic for getting there powers, one could say one time when the Bifrost was used to transport some one to earth, the fantastic fours space ship was caught in its path, and they got struck by cosmic energy from the Bifrost thus gaining their powers.

As for when the Fantastic four gained their powers and where the were up to through Endgame , I have four potential explanations, 1) like before they were put in comma, 2) they were specifically hit by the final use of the Bifrost , when Heimdall transports Hulk back to earth, and they all disappeared in snap 3) like last one but sue and reed Richards disappeared in the vanishing. The two of them are the heart of the team, them disappearing means no team is formed. As for what Ben and Johnny due during the 5 years and through endgame, in the comics, after Ben Grim gets his powers he is unhappy and views himself as a monster, and wants nothing more than to find a cure fore his powers, and with out there support of the rest of the team like he had in the comics, in the MCU he went into hiding. Meanwhile Johnny has so much grief from losing his sister becomes a hermit of some sort till she is brought back in endgame. 4) they could have got hit by the Bifrost at any moment in the MCU, got trapped in it at a single moment of time, which is frozen, meaning in the Bifrost they don’t age or experience time. as to how they are released: either they released with destruction of space stones , vessel follows emergency protocol and safely returned to earth 22 days after the first snap, at this point the explanation as to where they were during endgame could follow either reason 2 or 3, and saying even in the Bifrost they weren’t safe from the snap; or the other possibility as to how they get released is that in the future the Asgardians rebuild the Bifrost powered by a similar energy but not an exact match to that of the space stone, and when it is used for the first time it is like a cosmic reboot for the Bifrost and everything that got stuck in it over the years gets released. That including the fantastic four who now have powers

https://marvelcinematicuniverse.fandom.com/wiki/Bifrost_Bridge https://marvelcinematicuniverse.fandom.com/wiki/Heimdall https://marvelcinematicuniverse.fandom.com/wiki/Stormbreaker

Edit: I know captain marvel isn’t mutant in comics, but the current miss marvel is so who knows.