After Infinity War, the next big team up will have to top it. And while there are alot of epic stories that can be adapted, I personally think an adaptation of Secret Wars could be the most interesting and useful.
Secret War had alot of characters. But the heroes basically consists of the X-men, The Fantastic Four, and The Avengers. But the the Avengers that are there are Captain America, War Machine(wearing Iron Man's armor), Hawkeye, Hulk, Captain Marvel(although its a different version then the one we are getting), Thor, and Wasp. Replace the X-Men with the Inhumans, and maybe even put in the Guardians of the Galaxy to make up for the loss of the Fantastic Four. They can even replicate the twist in the comics by putting a popular bad guy on the heroes team.
Although most of the villains haven't been introduced to the MCU, getting Loki to take the place of Doctor Doom would put one of the best villains so far in charge of a team of super villains.
Although Secret Wars doesn't have a really deep story, it mostly involves alot of skirmishes between the two sides. But one of the most important scenes gave Spider-Man his black suit for the first time. I don't know how many movies Spiderman will have had by the time phase 4 ends, but this could lead to an angrier, more violent Spiderman and that would mix things up a bit. Then after a movie with him in the black suit, he can separate with it and venom can be introduced.
And while the bigger heroes are away, the sidekicks and minor heroes will be left to defend the earth. Imagine the Netflix characters having to team up to stop an alien attack while the Avengers are off on battle world. And if it is a two part movie, we can see two or three other movies hit showing heroes up to their necks in bad guys and none of the other heroes to help them. This would be the perfect time for odd team ups of characters you wouldn't normally see in each others movies, like Falcon and Black Pather, or Black Widow and Scarlett Witch.
And although it wouldn't offer much more then the two Avengers movies that have come out so far, it definitely wouldn't hurt to see two teams at war for a whole movie.
After a big event like this, many of the characters will be different than they were to begin with. I mentioned Spiderman above, but another thing that can come out of this is the Illuminati. In the comics, this is a group of some of the leaders of the different groups from around the world. This is the perfect time for the group to form in the Cinematic Universe.
And the Hulk can also be affected by this. It could affect him to the point where the other heroes feel the need to shoot him into space, which will lead to Planet Hulk and World War Hulk.
But this is all my opinion, and feel free to share your own in the comments below.