In the Avengers: Endgame, fans see the invincible Iron Man die when he snaps the stone saying “I am Iron Man” this results in killing both, Thanos and Iron Man. The scene is quite emotional as fans see Tony Stark embracing his own death like the hero he is.
In a podcast Kevin Smith said:
“As the new kids are coming, they’re still supported by the veteran players with storylines that tie into the sh*t that they made us fall in love with ten years ago. Unless this is part one of the buildup to Galactus. I mean, we know Kevin Feige referenced the Fantastic Four on that stage, though nothing specific. If you’re going to build to another big bad, who badder than the World Eater himself?”
Galactus was born a normal man by the name of Galan, being the last human of the universe that existed before the current Marvel Universe. He became the living entity of that previous universe and witnessing the big bang that created the Marvel Universe.
However, we believe that Marvel will not completely introduce Galactus immediately, it is expected that the character will be developed gradually and featured in the next wave of movies to build up a great hype. Therefore in the upcoming Fantastic Four movie we might get some intro to Galactus.
As per the comics the extension of the Avengers series might not be called the Avengers at all, we might be seeing a string of heroes such Captain Marvel, Black Panther, Monica Rambeau, Blue Marvel, and America Chavez and that is where we expect Galactus to feature as a prominent figure.
A fan theory also suggests that Thanos created Galactus to ensure that even after him balance of life in the universe exists, and who could be better than the Galactus (a.k.a) the Planet Eater for this job!