When the Guardians of the Galaxy went to Knowhere to meet Gamora’s buyer, audiences were introduced to Taneleer Tivan, The Collector (Benicio Del Toro), whose sole MO is to collect the galaxy’s unique relics, fauna, and species as part of his collections. In his museum, he had Cosmo the space Dog (please let him be in Guardians 2 and have telepathy), Howard The Duck, A Dark Elf, A Chitauri soldier, the Slither space slug, and even possibly Beta Ray Bill and Adam Warlock. He actively sought out the Power Stone, and offered to purchase Groot upon his death. Needless to say, he likes to collect things.
It was already announced that Benicio Del Toro would not appear in Guardians of the Galaxy Vol 2, as the actor would have been busy filming Star Wars: The Last Jedi as one of the main villains. That means it probably would have been the last time we would see him in an MCU film. However, Josh Brolin, whose playing Thanos in the upcoming Avengers Infinity War movie, announced via an Instagram Live video that he’ll be working with Del Toro, as well as Guardians Chris Pratt and Zoe Saldana in the upcoming film for a scene, which is filming right now in Atlanta.
Right now, we don’t know who the Collector is working on in the film. Is he going to be working for Thanos? Remember, when he first gained the Reality Stone in Thor: The Dark World, he said “one down, four to go.” Perhaps he is trying to gain the stones for himself, or like Ronan The Accuser, he is one of Thanos’ minions trying to get the stones for him. We’ll just have to wait and see in the upcoming film.