5. Star-Lord's Dad Is A Villain As Well
Also listed on that Marvel Movies fan page is the rumour that Kurt Russell's Ego The Living Planet, confirmed to be Star-Lord's long lost father, is the true villain of the film. Digital Spy seem to support the idea, suggesting that even though James Gunn is changing up the identity of Peter Quill's old man from the comics, Ego might "retain his villainous ways" as "a dangerous megalomaniac."
Official statements about the character haven't mentioned which side Ego is on, or how exactly he will impact the plot, which could be a clue in support of this theory.
For example, James Gunn described Russell's character to MovieWeb, in a rather vague fashion, as "an ancient cosmic being who has been on the edges of the universe for aeons by himself". That much time away from the society sounds like a classic ingredient in a supervillain origin story.
Ego's allegiances in the comics have shifted over the years, so it wouldn't be sacrilege to have him be a villain here. It would also tie in nicely with the first film's themes regarding friends-as-the-new-family. If Peter's dad turns out to be evil, it'll only strengthen Star-Lord's bonds with his teammates.