Back when the trailers for Guardians of the Galaxy were released, everybody believed that the Collector will have a pretty consistent role. It wasn't so - the movie (and James Gunn) used him just as an expositional device, to shed some light on the history of the Infinity Stones. As far as we are concerned, this wasn't the way in which a director can make the most of Benicio del Toro.
By the end of the movie, what we knew was that the Collector's base was destroyed - and that is just about all. So having him appear in the sequel made sense, to some degree. But it won't be so. Coming Soon reports that, after catching up with the actor, while he is promoting his latest flick, called A Perfect Day (to be released on the 15th of January).
When asked if he will return in Guardians of the Galaxy 2, the actor said, "I don't think so. They would have told me, I hope"¦ I do hope to play that character again at some point." This pretty much sums it up - even if, for example, IMDB lists him as returning, this won't happen.
But the Collector will be back, somewhere down the road, before the first Infinity War arrives (on the 4th of May 2018). How come? Well, he is in the possession of one coveted Infinity Stone, delivered to him in the post-credits scene of Thor: The Dark World. And, if we know something, Thanos will need that stone as well for his Gauntlet.
So when could he pop-up next? Since the Doctor Strange, Spider-Man, and Black Panther flicks will take place mainly on Earth, the Collector will certainly appear in Thor: Ragnarok - at least, this is what we hope for. And maybe this time they will use him (and the actor) in a proper way.