Some Avengers 4 predictions...
Steve Rogers is going to doubt himself after his two best friends (Bucky and Sam) are now dead, he failed to save Vision, his leadership failed, etc. His arc is going to be truly becoming Captain America again (complete with Tony giving him the shield back) and getting his mojo back.
Captain Marvel is going to be a pivotal character, a major force in the movie that stirs things up. She is going to take a leadership role (likely in the absence of Steve who is going to be too emotionally wrecked).
Shuri is going to bring Vision back from the dead, and he is going to be in his grey/white form, and without the Mind Stone he won't be as powerful but will probably have Wakandian Tech enhanced abilities.
Ant-Man is going to be a major character in the movie, and cement his place as an Avenger.
The third act is going to be a truly united Avengers front taking on a fully powered Thanos. The Avengers who got Snapped out of existence will get revived in the middle or just before, somehow, and Thanos will have to fight them too (Thanos vs. Everyone, basically).
Loki and Gamora are going to stay dead.
Thanos use of all the stones is going to be causing the universe to die off, which will create a ticking clock that motivates the Avengers to stop him beyond just wanting to undo the Snap.
Iron Man and Captain America are going to sacrifice themselves to save the universe in the third act.
Captain Marvel and Black Panther are going to be the main leaders of the Avengers going forward.