...The Collector. At least, according to this article from Comic Book Movie:
While the Academy Award-winning actor's lead role is unknown, his character is apparently “built to be part of future Marvel films,” which leads to speculation that The Wolfman actor is playing Thanos. However, a very reliable source confirms that Benicio Del Toro will really be playing Taneleer Tivan.
Better known as The Collector, one of the oldest sentient beings in all the cosmos, Taneleer Tivan's inclusion in Guardians of the Galaxy isn't exactly news. British actress Ophelia Lovibond was revealed last month to be playing a character in the film who “aides” Tivan. However, now that we know it's The Collector who's being set up for later appearances in the Marvel Cinematic Universe, besides Guardians sequels, could Benicio Del Toro be reprising the role in The Avengers sequel?I'm still hoping he'll be Thanos, though.