The Kree

The Kree are, of course, already an established presence in the MCU and, just like in the comic books (in which they first appeared in Fantastic Four #65 in 1967), they are the beings responsible for the creation of the Inhumans, through their experiments on humans thousands of years in the past.

It is known in the MCU that the Kree had already decided that creating the Inhumans was a mistake and had abandoned them and, in Agents of S.H.I.E.L.D., they even sent a Kree warrior called Vin-Tak to Earth to eradicate the newly created specimens (like Skye, for example), in the episode Who You Really Are.

When Vin-Tak was prevented from completing his mission by Phil Coulson’s team, he explicitly stated that the Kree Empire would return to eliminate the Inhumans if they were to find out that the experiments they had carried out were a success.

This opens the door to their return and seeing a war between two species – creators and their creations – in a movie would be pretty cool.