"Guardians of the Galaxy Vol 2" will be the movie that puts all of Marvel Phase 3 movies into perspective. We do know one of the greatest villains of the entire universe, Thanos, will be the villain in "The Avengers: Infinity Wars", and that could be one of the reasons that require the Avengers to seek the help of the "Guardians of the Galaxy".
However, if it doesn't, how could "GOTG" be involved in the Avengers' fight against Thanos? It could be the villainy of one evil Mistress Death. As fans of the comic book universe already know, this character is the literal embodiment of death, and Thanos is in love with her.
It is to impress her that Thanos wants the infinity stones. He believes if he wipes off half of the universe with just the snap of his fingers — by possessing the infinity stones — Mistress Death will finally fall in love with him. Even Josh Brolin, who plays Thanos in the Marvel Universe, has spoken about exploring the Thanos-Mistress Death story-line during interviews.
Another villain whose name has been floating around is Ego — the living planet and one of the main adversaries of Thor. He was just a normal rock in space when The Stranger, a being made of the souls of a 1,000 beings, gave it consciousness. As expected, the talking planet decides to take over other planets.
However, many fans believe being a planet does not allow Ego to be connected on a human level, and therefore he may just be a pawn, and the real villain of "Guardians of the Galaxy 2" would be his creator, The Stranger himself.
Meanwhile, if the goal of "Guardians" is to connect to the Thanos storyline in "The Avengers", Thane could also be the big bad. Thane is the son of Thanos, and an Inhuman. He grew up knowing nothing about his father and worse than him. Thane could be a proxy Thanos, or could be the reason Thanos is so insistent on collecting the Infinity gauntlet. Thane could also be used to foreshadow "The Inhumans" movie, which will be released in 2018.