I can't say that I would be surprised with your prediction. I think that they kinda need a powerful ally, though. The GotG are a pretty awesome group, but they lack a heavy hitter and it's part of why #2 was awful to me. They took on a Celestial with a half-Celestial who just learned a semblance of his power and a rag-tag group of comedians. It was just so far from tense.
We usually see a new villain in each film, so I would suspect he'd kinda be introduced as such. Maybe switch sides after a bout with Thor and team-up against a common threat.