I think it may be hard to make a part 2, which is why they went with a prequel before. I don't necessarily think it's impossible - the Laugh Floor concept comes after the events of the movie. They could do a story about a some human kids brought over to harness laughter who wander off and get lost in areas of Monstropolis. I mean, it'd need a strong story to go with it, but I'm not one to flat-out discount the idea. It's not what I'd probably want to see Pixar concentrating on though.