I'm thinking, oh no! I can't think of a single Disney sequel that didn't undermine the original in some way, or at the very least was much, much less good.
And Monsters, Inc is one of my fav Pixar, and its ending was perfect. I'd rather have it left as it is.
I'll wait and see what they do with it though. If Boo is an adult or a teenager for instance, it could be interesting. Also, seeing if there is more awareness of the Monster world. Maybe there are even more contacts between the two.
Still, my first reflex is to be very sad about it. I hope I'm wrong and that it will be the first amazing Disney sequel.