Super-Skrulls

Just like Quicksilver and Scarlet Witch, Marvel can use the Skrulls in its movies (even if they haven't appeared just yet). However, despite being one of the three major galactic empires from the comics, the Skrulls aren't exactely a match for the MCU superheroes "“ however, this cannot be said about the Super-Skrulls, a faction of this race that matches in power the Fantastic Four.

In this way, the Super-Skrulls are shapeshifters (like the Skrulls), but they also possess the immense powers of the Fantastic Four and other superheroes (in this latter case, telepathy), via artificial augmentation. So they could prove to be terrific against anything the Avengers or the Guardians of the Galaxy could throw at them.

The relationship with the other galactic empires is also an interesting one and, if Marvel Studios decides to go down this route, it could switch the focus away from Earth. Even more, given the infighting in the Skrull Empire, the Marvel Cinematic Universe could easily become the space-opera heralded by the first Guardians of the Galaxy movie.