3. Titannus
Although Titannus is a Skrull - a name that is apparently owned by Fox for the Fantastic Four franchise - he is by no means a Fantastic Four villain so there's no real reason why he couldn't appear in the Marvel Cinematic Universe. Lacking in the Skrull's usual shape-shifting abilities, he became one of the subjects of the Super-Skrull project, which gave him enhanced strength and a healing factor that would allow him to recover from almost any wound. After leaving the Skrull's home planet, Titannus eventually arrived on the planet Trellion. There he was brainwashed by the inhabitants to act as their agent. Believing that he was fleeing an oppressive ruler, he escaped and travelled to planet Earth in an attempt to seek the aid of the heroes so they could revolt against Trellion. After crash-landing in Japan, Titannus observed the heroes of Earth for several months and attempted to "gain their attention" by destroying Tokyo, defeating Japan's main superhero, Sunfire, and killing countless soldiers of the Japanese army. Of course, this did get the heroes of Earth's attention - but not in the way he planned it. Doctor Strange assembled a team of Defenders - which included existing MCU hero the Hulk - to put an end to his actions. Something like this would work great in the Marvel Cinematic Universe, with the Avengers being forced to stop him. Titannus possesses superhuman strength, stamina and durability, flight, energy blasts and a healing factor, making him a legitimate threat to them all.