8. The Phalanx
In the comic books, the Phalanx are a cybernetic alien species. They form a hive mind, linking each member by a telepathy-like system and have worked under Ultron's leadership on a couple of occasions. If it is Ultron's plan to take over the world in Avengers: Age of Ultron (which it most likely is), he could do worse than recruit their services, given that the Phalanx can infect other organisms with a virus through physical contact that causes the victim to join their forces and become a part of the Phalanx hive mind. Although the alien army thing has already been done in an Avengers movie (with the Chitauri in the first instalment), it's never a bad thing for a villain to have flunkies to keep the heroes busy while he goes about his business and the Phalanx would offer a new kind of threat to MCU Earth, given that they can grow their army using the planet's own inhabitants. They're also very cool looking and a live action CGI adaptation could be visually stunning.