It's' learned Adam Warlock almost made his way into The Avengers: Infinity War.
Jim Starlin included the character as a big part of his Infinity Gauntlet epic comic, which is somewhat of an inspiration for the movie.
The Avengers: Infinity War writers Christopher Markus and Stephen McFeely talked with ET Online about writing the Infinity War script and were questioned about characters that didn't make their way into the film:
McFeely: That we didn't end up? Yes.
Markus: Well, there are some in the original Infinity Gauntlet series that are-- Like, we didn't introduce Adam Warlock, because it's a massive backbend and you practically have to make an Adam Warlock movie to introduce him. He just can't walk onscreen. So, there was a brief moment where we were like, Ehhh..., and then we were like-- [He points to the movie's poster.] We have that many characters already! And then there were others we couldn't use. Like, Silver Surfer would have been useful, but we can't touch him. Currently.
McFeely: I don't want to spoil anything, because then if I tell you that they're not in it, you'll go "Oh, they're not in it." But there are certainly a handful of pretty crazy things that we put in for several drafts and took out.
I think the writers made the correct move not including Adam Warlock into Infinity War, as the generally audience would have been pretty clueless when and why he appeared. It also seems as if Infinity War isn't about introducing new characters, but about closing the door on the past ten years of the MCU, so with all of the characters already in Infinity War, having to explain Adam Warlock would be too much.
It's interesting they state Silver Surfer would have been useful, but at least the good news is that the character will be returning to Marvel Studios next year.
Regarding Adam Warlock in the MCU, James Gunn did include an Easter Egg for the character in Guardians of the Galaxy 2, so maybe HIM will appear in GOTG 3, or even somewhere down the line as part of Marvel's Phase 4 plans, which are said to be going COSMIC.
As the Avengers and their allies have continued to protect the world from threats too large for any one hero to handle, a new danger has emerged from the cosmic shadows: Thanos. A despot of intergalactic infamy, his goal is to collect all six Infinity Stones, artifacts of unimaginable power, and use them to inflict his twisted will on all of reality. Everything the Avengers have fought for has led up to this moment - the fate of Earth and existence itself has never been more uncertain.
Anthony and Joe Russo direct the film, which is produced by Kevin Feige. Louis D’Esposito, Victoria Alonso, Michael Grillo and Stan Lee are the executive producers. Christopher Markus & Stephen McFeely wrote the screenplay. “Avengers: Infinity War” releases in U.S. theaters on April 27, 2018.