In Monsters Inc, Sulley.
In Monsters University, Mike.
That’s a perfect observation. You can always tell by asking who has the biggest character arc. Mike didn’t really “change” in Monsters, Inc.
While that works here, it’s not a norm.
Flik doesn’t change and still he’s the main character in A Bug’s Life. He makes everyone else change instead.
Okay, “usually”. 
I suppose in these terms, it’s the character who has the most effect on things or is followed the most.
Sullivan’s interactions in MI were the driving force for what happens.
Wazowski’s interactions in MU were what started and ended things.
And Randall’s interactions will be what goes into the sequel.
If there is a sequel. I really hope Mike and Randall can become friends again.
I personally hope not. That would seem like fan service.
The idea is pretty sweet but I can’t see it working. Assuming Randall found some way out after getting beaten with the shovels, I think friendship with the monsters who put him there would be the last thing on his mind 
I don’t think so much a fan service SoA…but yeah, Leirin’s right…
Randall is…not going to be so forgiving. These three (him, Sullivan, and Wazowski) have been the victims of massive misunderstanding.
Wazowski was so obsessed with becoming a scarer in MU that he ignored what was going on in Randall’s life (who was more interested in being Wazowski’s friend rather than study partner).
Sullivan is unaware that what he did at the Scare Games damaged Randall’s ability to put trust in others.
Randall is unaware, because of his distress at the time and leaving, that Sullivan DID undergo a positive change. He’s got the image that Sullivan is STILL a jerk and a cheater, and hasn’t learned (because he didn’t GET the experience that Sullivan had with Wazowski, he doesn’t know what happened between the two. And that’s another thing…Wazowski, even though it was one-sided, was his first friend. And suddenly he’s off with the jerk behemoth that made fun of BOTH of them? That’s something he was unable to understand.)
I’ve been running sequel ideas in my head for years now. Sullivan and Wazowski, surprisingly, are simple enough. Randall’s the one that will take some work.
He’s going to be pretty injured from his experience no matter what, that’s assured. He’s probably been slated as a criminal “on the run” by the CDA with Sullivan and Wazowski free thanks to a cover up…
However Randall GETS back…his emotional state at the time would go completely nuts when he realizes the two got away with what they did to him, AND were even rewarded. I can imagine him staring up at a picture of MI’s new CEO with complete shock.
One idea I thought of was that, in that sort of moment, he just…gives up. He’s lost his ability to trust others, lost his job, lost any respect or admiration (thanks to the “on the run” thing). The only thing that he had to keep himself going was revenge. BUT, when he sees that the two had managed to get rid of him (and after a year, seems nobody cares they did), and were even rewarded…the track record speaks for itself. He loses, they gain.
Randall’s a survivor, like a lot of people. But in a moment like this he might come to the conclusion just to give up. He honestly can’t win against these two, for whatever reason. The only thing he has left is his actual life, and from how he feels, that ain’t worth much.
This a reason a sequel is needed. These three have…there’s a LOT of misunderstanding and a bunch of wounds that need to be fixed between them. They should ALL be happy, somehow.
Randall…will probably not trust them, probably never forgive them, but he may not grudge-hold it forever if something GOOD happens for him.
They all seriously need to be sat down and talk things out. As we saw in MU, it takes a while for Wazowski and Sullivan to get their feelings out, and it’s going to be way tougher for Randall. But, guess it’s a guy thing. We talk through actions, which aren’t as clear, rather than words.
In MI, Sulley was the main protagonist, but in MU, it’s mostly about Mike. We see him as a child and how he aspires to be a scarer.