Pixar started off as a studio that told only original tales, and while they still do that fairly often, they've also taken several opportunities to revisit their previously introduced characters in sequels. However, in 2013, they went in the opposite direction with Monsters University, a prequel showing how Mike and Sully met at monster college years before Monsters, Inc. Considering how that first movie ended, there's certainly room to follow up on what happened afterwards, but apparently the Pixar minds deliberately avoided making a sequel last time in order to not revisit the main human protagonist, Boo.
When asked by EW whether there are any plans to make another Monsters, Inc. movie, specifically one set after the events of the first movie, Pixar director/producer Pete Docter said the studio released Monsters University instead because they didn't feel there was a good enough reason to delve back into Boo's life. He said:
We purposely went with a prequel for Monsters University because we didn't want to answer some of the questions about what happens to Boo, and how does she grow up, and things like that. It would have to be really compelling, which is hopefully the benchmark for all of our sequels, anyway.
At the end of Monsters, Inc., Mike and Sully successfully returned Boo back to her bedroom, but her door was shredded so that there wouldn't be any more escapes. However, thanks to Mike's collecting all the door pieces and putting them back together, Sully was able to revisit his young friend again. It was a heartwarming moment, and no doubt there are many fans nowadays who are eager to know what happened to the girl. Assuming the same amount of time has passed in the Monsters world as it has for us, that means Boo (or whatever her real name is) would be in high school right now. Have Mike and Sully kept in contact with her all this time, or did they leave her be once she grew older, and now any recollection she may have of them she attributes to being imaginary?
While it would be amusing to see the teenage Boo reunite with the colorful creatures, as Pete Docter said, the Pixar minds wouldn't tackle that story unless it was truly compelling. As it stands now with if we'll ever see Monsters, Inc. 3, Docter simply responded "Who knows what will happen?", but there are currently no official plans to for a threequel. If you want more Monsters goodness, you'll have to delve back into the past and re-watch Monsters University.
However, there certainly aren't any shortage of other Pixar sequels on the horizon. Cars 3 hits theaters on June 16, 2017, The Incredibles 2 was recently moved to June 15, 2018 and Toy Story 4 will arrive on June 21, 2019. Finding Dory was also released this past June, and is now available on Digital HD (with the Blu-ray/DVD copies coming on November 15).