A recent fan theory of Avengers: Endgame suggests that the original Avengers will sacrifice themselves for the sake of the newer additions to the Avengers franchise. This will give Marvel the chance to focus on the new characters such as Captain Marvel and Black Panther.
It is widely known that the actors playing the original Avengers are at the end of their contracts with Marvel. There has been rumors that the original cast has been bidding farewell to the franchise while finishing up work on Endgame. Actor Chris Evans had said in an interview that his contract along with Downey’s and Hemsworth’s were all done at the end of Avengers 4.
Marvel is already looking ahead with plans for a stand alone Black Widow film already in the pipes. The film is supposed to be set before the events of the first Avengers film. The new theory might also help Marvel turn the Winter Soldier into the next Captain America and likewise follow the comic books for the next Phase of the MCU.
In the new theory, the new additions to the Avengers stand together to fight like the Guardians stood against Ronan the accuser. The original Avengers step in to save them and give their lives to bring back those that there killed at the end of Infinity war. The death of the original Avengers will end Phase 3 of the MCU in a very emotional manner.