Sully and Mike will return but not in the guise of a sequel but a prequel. "Monsters University" will show us how Mike and Sully entered the academy and overcame their differences to become the friends we know and love. Expect a hate-to-love buddy movie with plenty of laughs.
Monsters Inc was yet another reason why modern animation is both astoundingly creative and very funny and it'll be interesting to show how these guys tangled up in the first place after starting out seriously disliking each other.