Fans were definitely sure that Adam Warlock was inside the cocoon spotted in The Collector's, uh, collection when we saw it first at the mid-credit scene of Thor: The Dark World. Hopes were crushed when we saw Adam being made in a sleeker casing during the end credits of Guardians of the Galaxy Vol. 2, but as it turns out, the first cocoon can actually still be connected to Warlock.
Gunn was asked about this on Twitter, to which he replied:
It actually makes a lot of sense, and I'm happy that Gunn didn't render the first Easter egg irrelevant by having a newer coccon for Adam in GOTG Vol. 2.
For fans who didn't know, Adam was supposed to feature prominently in Vol. 2, but Gunn thought that there were already enough characters that needed to be covered in the movie including Mantis and Yondu. The Sovereign would have built Adam to chase after the Guardians, and it was likely that he would be part of the clash with Ego in the third act—in place of the Sovereigns remote-controlled drones.
For now, Adam is definitely confirmed for Guardians of the Galaxy Vol. 3. Don't expect him to appear in Avengers: Infinity War to clash with Thanos though.
Infinity War is set to release May 4, 2018.