8. The Void
In order to introduce the Void to the Marvel Cinematic Universe, the heroic character Sentry (Robert Reynolds) would first need to be introduced, since it is the alternative, evil side of his personality. The Void exists due to Reynolds covering up his shady past as a thief and a drug addict, which consequently makes him project an evil entity as an unfortunate dark side effect of his powers. In the comics, the Void was so physically strong that it was capable of breaking every single bone in the Hulk's body in seconds. Its most basic physical assault required the combined efforts of Iron Man's, Doctor Strange's and Invisible Woman's shields to keep it at bay and even then it was a struggle - and that was while the united forces of the New Avengers, Fantastic Four, X-Men, Inhumans, Illuminati and heavily armed S.H.I.E.L.D agents simultaneously attacked it to absolutely no avail. It also managed to take down Asgard by itself, defeating everyone who opposed it including an amped, Norn Stone-powered Loki with complete and utter ease. The Void can also shape-shift, allowing itself to appear as anything from a black, demonic humanoid entity in a trench coat to a massive, black, armoured monster. It also seems to be able to conjure powers that are related to its current shape - such as when it assumed a flame form and breathed fire. Without getting in to the abstract cosmic entities that are the size of planets, the Void is one of the most powerful villains in the Marvel comic book universe, and it would make for an awesome on-screen villain - it would look incredible utilising modern CGI.