Ant-Man

Scott Lang is the reason the Infinity Saga has an ending. The time heist in Endgame exists because he walked out of the Quantum Realm five years late with a working theory, and the film is honest that this was luck rather than genius.

He is also the franchise's most reliable audience surrogate: a man who is visibly astonished by the people he works with. In an ensemble the size of Doomsday, that function becomes more valuable, not less.

His Quantum Realm experience is the specific thing that makes him relevant to a multiverse story, and it would be strange for the film not to use it.