In about four years Avengers 3 is coming out and the year after that the next one and hopefully the Guardians will show up. But how?
So recently we've got the amazing news that Robert Downey Jr. is confirmed to star in [Captain America: Civil War](movie:994409).
My next point, Thanos is most likely to be the main antagonist of Infinity War and the Guardians have a certain hatred for that guy, so it would only make sense for the heroes of one of the best films of 2014 to show up Avengers 3: Infinity War.
But why would they go to Earth I hear you ask?
Well, I have a theory. In the civil war story arc Captain America and Iron Man don't exactly get on ad in the comics they have a punch-up and that's how The Guardians are going to show up. Captain America and Iron Man are going to have the climatic fight at the end of Civil War and Iron Man is going to win because you know the whole protected but a gold-titanium alloy thing, and he is going to think he has killed Captain America (But he hasn't, because you know Chris Evans still has contracts.). Racked with guilt he flies off into space because let's face it he's probably upgraded his suit to space status after the battle of New York.
So the [Guardians of the Galaxy 2](movie:1081113) comes around and after they go on their second adventure there's an end credit scene in Knowhere in that bar where Rocket is all drunk in the first movie. It goes like this. There's a dark haired guy sitting at the bar with a red and gold suitcase, Peter Quil and Rocket goes up and says "Nice case." and the guys turns arounds and goes "Thanks nice... I want to say Raccon." and it's none other than Tony Stark. BOOM! black screen. Then Infinity War Part II, after Part I the Avengers are scattered someone's probably died. Then when a certain purple tyrant is about to kill Steve Rogers, Iron Man rolls up with the Millano and shoots the certain tyrant giving Steve Rogers enough time to get away. The Milano flies away and Iron Man snatches up Steve Rogers and flies away to wherever the Avengers have made base. Then the rest of the film happens and stuff.