6. Blood Brothers
With Thanos undoubtedly on the verge of an appearance in the Marvel Cinematic Universe, the likelihood of him turning up without henchmen (going by the comic books, at least) is very slim. Therefore, the Blood Brothers - ape-like aliens who originally appeared on-panel in the service of the Mad Titan - would be a welcome inclusion alongside him. Hailing from the planet Rocklon, they were essentially the "bouncers" at the very first base Thanos ever made on Earth, so it stands to reason that their inclusion in his first major appearance on Earth is extremely plausible. In the comics, when Iron Man and the Thing (who couldn't appear in the Marvel Cinematic Universe, but could be replaced by another character) tried to interfere with Thanos' plans, he sent the brothers to kill them. Perhaps, in a movie, he could send them to carry out the same task after the Avengers have interfered with his plans to take over the Earth. The pairing certainly aren't capable of carrying the villainous responsibilities on their own in a movie, but they would be an interesting sub-plot alongside Thanos and they'd certainly be unlike anything seen in the Marvel Cinematic Universe to date.