With the end of the Phase 3 in the Marvel Cinematic Universe, (MCU) Marvel must find a way to continue the threat and momentum of what can arguably be the greatest sets of sequel movies of our time. To me the biggest question to ask when looking at the MCU is who or what can be the big villain for Phases 4-7.
When thinking about the upcoming MCU movies. It seems Marvel is moving much more into a cosmic realm with movies like Doctor Strange, Captain Marvel, Guardians of the Galaxy 2 and Inhumans. What I think this means is that the next big villain for MCU will be something in the cosmic realm or universe. To me, this leaves the MCU with three major choices: Kree, Skrull, or Kang.
Now while I don't honestly know much about the Kree, it is clear that quite a bit of the Kree lifestyle is set up in GotG. This could be Marvel getting ready for some type of face off with the Kree, however I don't think just the Kree alone would be enough of a bad guy for multiple movies, even though they were good villains in Guardians of the Galaxy.
I would love to see a secret war/Kree Skrull war in the MCU. I think this is one of the next big logical and amazing story lines that could be told in the MCU. The secret war story line allows for a lot of room to play with story lines and characters. The idea of superheros not being who they appear to be is very compelling, and the secret war story line also allows the MCU to bring back heroes that die. Maybe Captain America dies in IW, only to see him brought back by mysterious means in a Secret War movie or phase build.
This would be my best choice for a new marvel villain filling the shoes of Thanos. If Kang were going to be the next big bad I would do the Kang storyline with Captain America. The story that Captain America should have died, and thus by being alive has somehow disrupted the space time continuum and therefore destoryed Kang's timeline/reality is breathtaking in my opinion. Pull this with the idea that Captain America is dead, and replaced by a Skrull in a secret war, and I think you have a classic MCU build. Not to mention the fact that it could be cool to see Cap brought back to life and given the keys to the Avenger Kingdom only to mess things up. Watching other heroes like Black Panther, and Ant-Man watch as Cap takes over only to screw up would also add a deeper level to what the Avengers is how they function.
Ideally with this type of story the MCU would do Phase 4 with a focus on the Kree and a Kree invasion. Then Phase 5 would be about the Secret War of the Skrull Empire building though to Phase 6 which would ultimately climax with a Kree/Skrull war and Kang the Conquer coming to Earth.
The biggest problem with this fan theory is that Fox still has the rights to Fantastic Four and thus, the Skrull race and maybe even Kang. Not to mention Doctor Doom who could possibly be as evil and deceptive as Loki was. It is for these reasons that Marvel must get the rights to Fantastic Four back. It really is a shame that Fox has done nothing but pump out horrible Fantastic Four movies. Not only does it give superhero movies a bad name, but it also just looks and feels...wrong. The reason it feel wrong is there seems to be no honesty in the stories that Fox is telling. When watching any live action Fantastic Four it comes off like a bad commercial for product placement, along with cheesy horrible superhero puns. Fox has been trying to make money off of the Fantastic Four for years and failing like a self consciousness fourteen year old fails at asking a supermodel to dance. With shame and disgrace. To quote Futurama, "Your movies are bad and you should feel bad."
Fox should just cut the losses, settle up and make some REAL money off a good set movies done by Marvel Studios. Rather than making some money off a bad set of movies and thus ruining not only the value of Superhero movies but the MCU itself by limiting the scope of the universe.