Quite some time ago, James Gunn said that additional superheroes would join the team in Guardians of the Galaxy: Vol. 2. However, there were so many characters to juggle that he was forced to drop one of them. At that time, given the film’s antagonist, as well as the impending Infinity War, we all assumed that the director was talking about Adam Warlock. The character was a member of the same race as Ayesha, and played an important part in the comic book version of the Infinity War.
As it turns out, Adam Warlock was indeed going to be featured in Guardians of the Galaxy: Vol. 2, and in an even bigger role than anyone expected.
Gunn told Slash Film that the superhero was to have a major role in the second GotG2. However, the director realized that he had to let one of the characters go. And, since Mantis fit more organically with the team, that character ended up being Adam Warlock:
I love Adam Warlock. He's one of my favorite characters. In all truth, I wrote an early treatment where I had Adam Warlock as a part"¦this is your big scoop, because I don't think I've told anybody this"¦It was Adam Warlock. He was a big part of the screenplay, and I realized it was one character too many. And I adored the character. I adored what we had done with him. I think we did something really creative and unique with Adam Warlock. But it was one character too many, and I didn't want to lose Mantis, and Mantis was more organically part of the movie anyway. So I decided to save him for later.
This means that Adam Warlock will eventually be introduced in the MCU, as so many fans wished after he was teased in the first Guardians of the Galaxy flick. However, Kevin Feige recently said that Warlock’s first MCU appearance could only be in the Guardians of the Galaxy franchise. So the superhero has already been confirmed for Guardians of the Galaxy 3 (we’re assuming that he won't appear in Avengers: Infinity War or Avengers 4)!
It remains to be seen how important the character will be in the MCU. Maybe he'll start as a hero and end up as a villain. We'd like that!