Iron Man (2008) gave rise to an exciting cinematic universe which is successful to date. One question every Marvel fan is asking, if all the three phases lead up to the successful comic event "Infinity War", what will Phase 4 be about?
The answer is obvious right? It has been right in front of our eyes, the whole time. Yes, Phase four will be either about "Secret Wars" or "Secret Invasion"
Though Agents of S.H.I.E.L.D is Coulson's show and it came after the first Avengers movie, one mustn't ignore the fact that it was due to Coulson, that the Avengers end up fighting as a team. Agents of S.H.I.E.L.D has done a pretty good job of connecting all Marvel movies at the base level.
Here is where things get interesting: We have seen the entire season 2 incorporating Inhumans. We all know that there will be an Inhumans movie in 2018, so it would be interesting to know if the TV characters would be coming to the movie or not.
Season 3 has confirmed the rumours that Secret warriors are very much coming to the MCU.
Now comic book fans know that Secret Warriors played an important role in Secret Invasion. There is a high chance we will see the inclusion of Skrulls in the fold of MCU. We also know that Kevin Feige is a person who wouldn't restrict Skrulls only to Agents Of S.H.I.E.L.D. So my best bet is that Phase 4 will start to lead to Secret Invasion.
My theory doesn't end here...
Guardians of the Galaxy introduced the Nova Empire and the alien race the Kree.
Agents of S.H.I.E.L.D. continued that connection in its season 2. It also ended in a cliffhanger.
Simmons was consumed by a Kree monolith.
The MCU is also notorious for not following the comic material as it is. It may adapt the Secret Wars involving the Kree so that it is suitable for the big screen.