A massive scene will have every character in the MCU on screen
While promoting Infinity War, Sebastian Stan gave away what everybody thought was a spoiler for Avengers 3. However, the scene that Stan spoke of was definitely not in the movie. Instead, it is believed that there will be a huge scene in Avengers 4 that will include everybody, even the characters who were killed off during the events of Infinity War, plus new additions like Brie Larson's Captain Marvel and Michelle Pfeiffer's Janet van Dyne. Stan had this to say about the gigantic scene.
"There was one scene, I think, we had where everybody was there. I can't really talk about that scene, but I knew it took them three months in planning this scene to have everyone there. You look around and you just saw everyone from Samuel L. Jackson to Michael Douglas to Michelle Pfeiffer. Everybody was there."
Captain Marvel will join The Avengers
Even though the Russo Brothers had told fans at the 2017 San Diego Comic Con that Brie Larson's Captain Marvel would not be in Infinity War, there were still several fans who were disappointed by her absence. However, she was alluded to during the post-credit scene where Nick Fury messaged her. Avengers 4 screenwriters Stephen McFeely and Christopher Markus have said that Carol Danvers will have a substantial role and that she will bring her military background with her, which will be needed. Again, Captain Marvel will tie into Avengers 4 in a meaningful way, according to McFeely and Markus.
Clint Barton will return as Ronin
Set photos from Avengers 4 featured Jeremy Renner, who was sorely missing from Infinity War, in a new suit that closely resembles his Ronin costume from the comics. In addition, there were some scenes that were filmed in Japan (Ronin is Japanese term for a masterclass samurai), leading many to believe that Clint Barton has been hiding out in Japan. Scarlet Johansson was spotted on the Japanese set as well, which may point to Black Widow traveling overseas to gain the help of Hawkeye or Ronin. His return will be a welcome one since MCU fans were so unhappy that he didn't make an appearance in Infinity War.
It won't reverse all of the events of Infinity War
Screenwriters Stephen McFeely and Christopher Markus have revealed that Avengers 4 will not reverse all of the events of Infinity War. The screenwriters have warned fans that the deaths will be real, which is something that the Russo Brothers also said before Avengers 3 was released. This has led to much speculation that the theories of time travel have been squashed and deemed too obvious. However, a new theory suggests that Thanos may have caused a second Big Bang, creating a parallel universe. McFeely had this to say about the deaths being final in Avengers 4.
"Avengers 4 doesn't do what you think it does. It is a different movie than you think it is. Also... (the deaths are) real. I just want to tell you it's real, and the sooner you accept that, the sooner you will be able to move on to the next stage of grief."