Remember a few days ago I was geeking out because I suddenly remembered that Thanos and Drax first appeared in
Iron Man #55? I conjectured that Marvel might be planning to send Tony into space because that's exactly how the whole Thanos epic kicked off in the comics. Now other sources are indicating that I might have been right in my speculations. I mention all this not merely to toot my own horn (yay, me!

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but also because the story in IM55 could give us a clear view of what will happen in GotG.
While the story in the comic book is a bit confusing, one thing is clear: Tony Stark ended up in space because Thanos' goons kidnapped him. The abduction happened after Drax contacted Stark telepathically asking for help. So there is the way Stark/Iron Man could get into space and become involved with Thanos and the GotG. The original can easily serve as a template upon which the films' writers can build and improve.
In the original story it's not clear how Drax, Mentor and Thanos became aware of Stark's existence and targeted him. The MCU has that covered since Thanos and The Other are already very well aware of The Avengers' existence. There are any number of plausible scenarios in which the Mad Titan might decide that capturing Stark would aid in his plans to conquer Earth and/or confront the heroes who defeated his army.
This is brilliant, IMO. It allows Marvel to stay true to the comic while adapting it to the events in the MCU. Although some fans who are ignorant of the original story will argue that Iron Man is being shoehorned into it just because he has name recognition, that couldn't be farther from the truth. It all flows organically from that first comic books story, through the Avengers (both in the MCU and the comics) and on to GotG. There is room for Captain Mar-Vell and Warlock, too. It's gonna be one helluva ride.