One knock on "It" is Mike doesn't get a lot of screen time compared to the other losers. But that will likely change in Chapter Two.
Mike is the one member who doesn't leave Derry. He stays to watch for the return of Pennywise while working as a librarian, and staying behind, he ends up taking the brunt of Pennywise's powers.
“My idea of Mike in the second movie is quite darker from the book,” Muschietti told EW. “I want to make his character the one pivotal character who brings them all together, but staying in Derry took a toll with him. I want him to be a junkie actually. A librarian junkie. When the second movie starts, he’s a wreck.”
Mike will also take part in a ritual that will bring him better understanding of the origins of the alien behind the Pennywise clown, and how the defeat it for good. This is a portion of the book that was cut out of the first movie, but will certainly be of importance in the sequel.