I hope the Guardians come into combat with the Collector, and think it's just a simple case of an overzealous completist. Maybe the Collector has abducted members of the original Guardians, and someone (Yondu perhaps?) has to gather together a motley crew of individuals to rescue the original team and stop the Collector.
The Collector may have amassed a whole load of Marvel goodies (similar to the various artifacts seen in Thor) and a bunch of other cosmic characters who have been placed in suspended animation.
Then maybe, in a twist (for the audience), it turns out that the Collector is actually trying to protect the Guardians and the universe from a much greater threat - Thanos.
You don't really get these villains in these Marvel movies who have more shades of grey. They tend to be more black and white and are usually outright villainous. Here's the chance for Marvel to do something different and offer up a villain who does have more altrustic motives, even though he's going about it in not the best of ways. Then the Guardians would be conflicted as to whether or not they should help him. Maybe they decide that they don't want to in the end, because freedom is freedom, and no-one should have that right taken away.
The Collector might say that they don't know what is at stake but the Guardians might ignore the warning. Then in the end, if Thanos arrives as the big bad, they'll realise that maybe he was correct. It would be a way to not have Thanos immediately be the villain of the movie, but his presence would be felt strongly throughout because you would know how connected the Collector is to him and what his motives are for collecting.