11. HYDRA
Although they never worked together in the comic books, Ultron and Red Skull could be being considered to form an on-screen depiction of the Masters of Evil in the future of the Marvel Cinematic Universe, so what about creating that connection by having Ultron working with HYDRA? Arnim Zola is set to return to the screen next year in Captain America: The Winter Soldier, so HYDRA certainly have a future in the MCU. Whether or not he will be human or some kind of cyborg like he became in the comic books remains to be seen, but to have him develop or assist someone like Ultron alongside other members of the evil organisation is very plausible. And, of course, the Red Skull didn't necessarily die in Captain America: The First Avenger - he seemed to be absorbed in to the Tesseract. If the Tesseract is something that Ultron is interested in (as all the MCU's big villains seem to be interested in the Infinity Stones), then it could pave the way for Red Skull to return in the future.