Sure we know the obvious answer with 4 and X. But I'm referring to the rest of the MCU like The Avengers. Spider-Man, Doctor Strange, etc.
Warning: There will be spoilers related to Avengers endgame I will discuss these after the jump for the sake of being nice.
Ok, so we know what happened to most of the Original Avengers in Endgame:
Steve: tampered with time and is now old
Only Hulk and Hawkeye remain.
So, what do we do?
Refill those spots.
We have Falcon as Captain America but since he wasn't officially made Captain America by the US government, the spot is filled by a substitute Cap... yes I mean US Agent. But this new cap happens to be a bit too radical and has a grudge towards Captain Falcon. Before becoming a u.s. agent he could wear the Bucky cap outfit.
Rhodey takes the mantle because the world needs an Iron Man. He struggles a bit with the responsibilities of the role because anyone can wear the armor but there is only one Tony Stark and he's dead. Not to mention the Ultron Army created by the small nation of Latveria.
Wild yes Chris Hemsworth may have one or two more movies to go, his contract ending is an inevitability. There are rumors about Beta Ray Bill showing up in Guardians 3 but he's not exactly the best replacement for Earth.
His replacement would be Thunderstrike, an avatar of Thor on Earth. Just how better Ray Bill would be an avatar of Thor in another planet. Now you may be thinking why would be for leaving avatars of himself in multiple planets? Well that would be for the character that I consider an even bigger threat than Thanos...
This obviously allows for the introduction of the 4. Their origin could be tied to the cosmic energy released by the Stark Snap.
They're a way for introducing elements of the Fantastic Four into the MCU without going straight out to a Fantastic Four movie that could also be used for certain mutants mainly Wolverine Xavier Magneto just to have the characters around before we get the full movies with them.
Victor von Doom can be easily used in an Ironman movie for the tech side and a Doctor Strange movie for the magic side.
Captain Marvel or the Guardians of the Galaxy could introduce some of the Heralds of Galactus. Hell, even the Starjammers could make an appearance with Captain Marvel.
Last but not least Spider-Man:
We know that Spider-Man disappeared with the snap. What if New York needed a Spider-Man and since the real deal had vanished due to the snap a new Spiderman was made? If Sony were smart they would do a clone saga movie with a few tweaks so we could have a different actor than Tom Holland playing the Clones or at least the Ben Reilly clone. That way Disney cuz dick around with the Clone but still have to lend certain important characters to the Sony movies... while Sony can do Spider-Man related movies both in and out the MCU. The dumb five-year Gap was what gave me the idea to have a small war between Ben Reilly and Kaine.