Red Skull

Although it appeared that the Red Skull perished at the end of Captain America: The First Avenger, the likelihood is that he was actually simply either absorbed into the Tesseract or sent to the other end of the universe by its power.

This theory is reinforced by the fact that the Tesseract is the MCU version of the Space Gem and, as such, by its very nature it moves and transports people. As a result – and with Thanos inevitably close to getting his hands on it – there’s every chance he could encounter Red Skull and utilise his services.

A revived Red Skull would be likely to want to double-cross Thanos as much as any other villain and his return could also allow for a re-emergence of HYDRA as a major entity in the MCU.

Moreover, his knowledge, resources and ruthlessness combined with Thanos’ power would make for one hell of a formidable partnership for the Avengers and their allies to overcome.