So now the question is this: Who is it?
Here's what we know. Gunn was asked which Marvel character he'd most want to film, and Gunn gave this response:
Impressive how he dodged the question by revealing he's adding a new character to the sequel, isn't it? Anyways, we can assume that this superhero is probably a cosmic character, given the material. And as /Film points out, Gunn's been on record professing his love for 1) Beta Ray Bill, the horse-faced alien who ends up be worthy of Thor's hammer Mjolnir, 2) Adam Warlock, the cosmic wanderer who has battled Thanos on several occasions, an 3) Starhawk, an alien hero who was one of the original members of the Guardians of the Galaxy.
Unfortunately, any of these are pretty solid guesses. Starhawk could simply be a new member of the team. GotG2 could briefly introduce Beta Ray Bill to the Marvel Cinematic Universe, which would be excellent timing given that his character plays a major part in Thor's Ragnorak storyline, which is of course the basis of the third Thor film. And Adam Warlock... well, he's more or less Thanos' arch-nemesis, and since Thanos has already been established as the main antagonist of the two Infinity War movies, well, the odds of Adam Warlock showing up sooner or later are extremely high. Plus, Gunn introduced Warlock's giant cocoon in the Collector's collection, but he's said that was just a random easter egg and not an intentional sign of things to come.
But we don't know whether this character is going to be a major part of the movie or a clever cameo. Beta Ray Bill could randomly pass by Drax at some point, or Starhawk could join the team in reel one. Frankly, these three characters all seem so reasonable that GotG2 could include more than one of them — maybe even all three.
All I know is if Marvel manages to put Beta Ray Bill in movie theaters before the Justice League film, everyone at WB/DC should seriously quit and go home.