Looking beyond the Infinity War and the big Battle with Thanos, one feels a sense of uncertainty about what Marvel's Cinematic Universe will be like post 2019.
In Phase 3 we will be introduced to new characters like Black Panther, Doctor Strange, Captain Marvel and the Inhumans. We may also see the ends of story arcs for Captain America, Iron-Man and Thor in the Infinity War. Depending on how their individual stories play out, we may have a different Captain America; as Bucky (Sebastian Stan) may assume the mantle once Chris Evans is done playing the Sentinel of Liberty. Marvel may even give Falcon (Anthony Mackie) the mantle of Captain America, since he is currently Cap in the Comics.
Robert Downey Jr's contract with Marvel ends with the Infinity War, so most likely we won't be seeing any more Iron-Man/Tony Stark past this point.
It is at this point that we may see new leadership emerge in the MCU with the addition of Doctor Strange- played by Benedict Cumberbatch. He may very well provide the mentoring and leadership in the absence of a Captain America or Iron-Man to the Heroes who may emerge in Phase 4. But the question remains... Who will be the new Heroes of Marvel's Phase 4?
Nova would be an easy addition to the MCU at this point in Phase 4. We're already familiar with Nova Corps in Guardians of the Galaxy (they will already be up to part 2 by this point.) He might already be introduced in the 2nd (GoTG) movie prior to Infinity War- so who knows? Point is, He's a powerful hero to have around.
Spider-Woman is another easy addition to the MCU. She's not associated in any way with the Spider-Man universe so there is no conflicts with Sony. As a S.H.I.E.L.D. Agent she could be introduced fairly easily on either the Agents of S.H.I.E.L.D. TV Show or in one of the other upcoming MCU movies- much the same way Black Widow was introduced in Phase 1. Her powers and espionage skills would be great assets to the surviving Avenger's Team of Phase 4
Another character that could be introduced easily. She is the cousin of Bruce Banner who needs an emergency transfusion of blood and Banner saves her. The flip-side to saving her of course is now she is a Green Rage Monster like her cousin Bruce (well... not really.) As She-Hulk, she still has her intellect and can transform at will. She-Hulk is another great character to have on the Team.
Those are my ideas for the Phase 4 additions to the line-up of The Avengers. Who will they battle in Phase 4?
It's time for the Super-Skrulls to begin their Secret Invasion!