With Marvel Studios' line-up of films, the casual comics fan can tell that a key piece to the future of the Marvel Cinematic Universe (MCU) lie in an alien race that has already been subtly introduced in [Guardians of the Galaxy](movie:424073) known as the Kree. With Ronan the Accuser (a Kree) having already given his people's war-torn history with the Nova Corps, we have a backstory to set their stage for future movies. However, the Kree seem to have a bigger role outside of the realms of the Guardians.
The two films being released between [The Avengers: Infinity War](movie:738027) Pt I & Pt II (Captain Marvel & [The Inhumans](movie:910490)) both owe their powers to the merging of human and Kree DNA. Captain Marvel's superpower origins lie in the fact that her DNA was merged with the Kree captain named Mar-Vell (hence the origins of her name), while the Inhumans were an ancient experiment by the Kree to create a new breed that they could use as tools in war, which they created by merging their DNA with the dominant Earth (Terran) species: humans.
So why is Marvel Studios choosing to introduce the Kree right in the middle of the Infinity Wars, which will undeniably center around the purple titan Thanos? After looking into some of the comic book history, I believe Marvel may have found its key to the Infinity Wars: Thanos' Inhuman son, Thane.
In the comics, Thanos had a son by an Inhuman woman. However, Thanos is deeply in love with Death. Death is an actual woman in the Marvel Universe. She instructed him to kill all of his children, and ideally a son could challenge his throne. So for years, Thanos has been traveling around to each planet demanding the slaughter of all their inhabitants ages 16-22, or he'll destroy your planet to ensure the death of the son he lost track of. This quest eventually leads Thanos to Earth.
The theory that Thane is the reason for Thanos' interest in Earth may seem far-fetched in theory, but examine the facts and the pieces fit together. The Inhumans is the last film before Avengers: Infinity Wars Pt II. In a conversation between Thanos' adoptive daughters Gamora & Nebula in Guardians of the Galaxy, Nebula says the following:
This seems to suggest that Thanos has only daughters, and only adoptive ones (considering his two favorites are both adopted, and are probably taken as trophies from the planets he's destroyed). Thanos also gives a huge smile at the end of The Avengers when told that battling the Avengers is to "court [AKA date] Death", who is the woman he's in love with, which essentially acknowledges her existence in the Marvel Cinematic Universe.
What do you think? Could Thane be the reason that Thanos returns to Earth in The Avengers: Infinity Wars Pt I & Pt II?