Oh, wow. I wonder if they're going to incorporate anything into the Avengers movies, considering the characters shared between them, or if they'll remain standalone. IIRC, in the comics the Guardians assist the Avengers in defeating Thanos or something, but I don't know what Marvel has planned.
Personally, I don't think you could fit the Avengers and the Guardians on the same screen without overcrowding the whole ordeal. Volleying Thanos as the main villain between several films starring the Guardians and the Avengers separately seems more likely.
Then again, Marvel tries to at least hold a little true to the comics, so who knows.