Guardians of The Galaxy Vol. 2: What Can We Expect?

 No. 1: New Team Members

Adam Warlock is an alien magician with enormous power; he has even taken control over the Soul Gem for a while. You know, one of the power stones that goes on Thanos' Infinity Gauntlet; the one that can give him rule over reality itself?

Stay with me.

Adam Warlock bore the Soul Gem on his forehead, just like the Vision did during Avengers: Age of Ultron.

So if the Guardians were to go up against Thanos, you know who they'd want by their side, because even without the gem Warlock could still can be a huge asset. Now, of course, Adam Warlock isn't a widely known character, but neither were any of the Guardians before the last movie, so it wouldn't be too much of a leap to see him join the team.