What Could Have Been: According to Dan Scanlon, the movie originally opened with Mike's parents dropping him off at the university on his first day there, and Billy Crystal actually voiced his whole family! It was cut from the film due to Scanlon wanting to remove parental guidance from Mike's college life. Hearing Crystal talk with himself would've been hilarious!
Other early drafts included Mike and Sulley in grade school together (to respect the "fourth grade" quote in Monsters Inc.) It reached storyboards process. Then Pete Doctor advised Scanlon to disregard the quote to stick to the focus of the University.
Characters of Celia, Fungus and Waternoose (recast as Kelsey Grammer) originally were to make appearances. Grammer's cameo was cut from the film; Waternoose appears only in a photograph.
Not surprisingly, the filmmakers tried to make it Sulley's story until they saw the story potential in Mike's struggles. Some concepts they experimented with was that Sulley initially wanted to be a dentist and Sulley suffered a case a Disappeared Dad. These scenes would have probably explored Sulley's relationship with the unseen Bill Sullivan.
Originally Monsters University was going to be a sequel to Monsters Inc.,and was called "Monsters, Inc. 2: Lost in Scaradise," which was created by Circle 7. It would've been about Mike & Sulley dropping by Boo's room to wish her a happy birthday, only to discover that somebody different is sleeping in Boo's room. It seems that Boo and her family have moved since Mike and Sulley last visited her. Sulley is so determined to see Boo again that he and Mike venture out in the human world in order to find her. Once Mike and Sulley cross over into the human world, they then aren't allowed to return to Monstropolis until they actually reunite with Boo. It would also include an older Boo trying to hide Mike and Sulley from other people.
Claire Wheeler was originally designed as an Eta Hiss Hiss girl until the animators thought it would be fun to place a goth character on the top of the Greek Life council. Even earlier drafts had her as a bubbly drama student.
The Oozmas were originally humiliated at a Movie Night.
Hardscrabble was originally supposed to be male and yelled alot until Scanlon decided to feature a great female Scarer that was more sophisticated and intimidating.
Old designs of Mike had him wear braces until Scanlon realized he would have to avoid the argument that "Mike would be scarier without braces" so Mike was redesigned with a retainer he could remove.
One ending they experimented with was after Mike's and Sulley's expulsion, Waternoose drives up in a limo, hearing about Mike's and Sulley's brief stint in the Human World and offers them jobs. The idea was unsurprisingly rejected as it would negate the whole theme of hard work.
Don Carlton originally had fire-breathing abilities and he was learning to control it.