Before Monsters University was a collegiate twinkle in Pixar's eye, Disney considered a Monsters Inc. sequel featuring not just Mike and Sulley, but Boo as well.
In 2004, Disney started Circle 7 animation, a studio designed to create sequels to Disney-owned Pixar films. When relations between Pixar and Disney were less than sunny, Circle 7 was Disney's possible backup plan. No completed films came out of the studio, although its employees worked on early scripts for sequels to Toy Story 2, Monsters Inc., and Finding Nemo.
Some of you may have seen this before, but back in 2011, screenwriters Bob Hilgenberg and Rob Muir released a few details and images from their never-produced Monsters Inc. sequel: Monsters Inc. 2: Lost in Scaradise. The movie was to be set some time after the first, when Mike and Sulley decide to visit Boo on her birthday. But Boo has moved away from the house attached to her closet door, and Mike and Sulley decide to look for her. Once they've decided to seek her out, however, they can't return Monstropolis until they've found her. The artwork linked to that early script appears in the the video above.
Hilgenberg and Muir also worked on a Toy Story 3 script, which comes with its own bounty of concept art. The pair have promised to chat more about Monsters University on their blog, so perhaps we'll get to hear more about their work with Circle 7 in the coming days. It's hard to say how this would have turned out, especially if it had been incubated outside of Pixar, but Mike and Sulley out in our world would have been a 180 from the delightful bits of worldbuilding we got in Monsters University.