I don't think I've read any Marvel comics other than "Demon in a Bottle", but I do know that Thanos is the big-bad in the Infinity Gauntlet storyline. We have seen 3 of the Infinity Stones so far in the Marvel movies (The Tesseract, the Aether [from Thor 2], and the Purple One [from Guardians]), and we saw Thanos at the end of the Avengers. Presumably, they are setting up the Infinity Gauntlet as a future story-line, which explains why they have a major actor in the role:

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