So far, we've seen the Collector - a man who, as his name would suggest, collects rare and powerful artefacts and species for his exhibit on Knowhere - getting his hands on two Infinity Stones. He took the Aether from out of the hands of Asgard at the end of Thor: The Dark World and he briefly had the Orb in his grasp in Guardians of the Galaxy. What we don't know is whether or not he has his own agenda with regards to those items or if he was collecting them for Thanos - either way, he is rumoured to be appearing in Avengers 3. Whether or not he'll be Thanos' underling or whether he'll be working alone remains to be seen, but he would certainly be an antagonist rather than an ally to the titular heroes if he did appear.
