Relevant notes from an M. Night interview that answer several questions posed in this thread:
ENTERTAINMENT WEEKLY: When did you strike upon the idea to set Split in the same story universe as David Dunn (Bruce Willis) and Elijah Price (Samuel L. Jackson) and Unbreakable?
M. NIGHT SHYAMALAN: Oh, it was always there. Always. This character, Kevin from Split, was in the original script of Unbreakable. The original draft of Unbreakable focused on David Dunn and Elijah as his mentor. Elijah tells him, “You’re a comic book character, go try it.” And instead of bumping into the Orange Suit Man, David bumps into one of Kevin’s personalities and goes to save the girls. So you’d have been watching the girls side of it the whole time. That was the outline.
Q: And how much of that screenplay had you written?
A: A lot. Some of the Kevin Wendell Crumb [James McAvoy’s character in Split] scenes were already completely written, all the way back then. One of the “Patricia” scenes, the Hedwig introduction scene, those were written over 15 years ago. I have them written by hand in my notebooks.
Q: So now hypothetically this seems like a perfect segue into a third film. That elusive Unbreakable sequel.
A: Yeah. I agree with that.
Q: Will it be?
A; I hope so. The answer is yes. I’m just such a wimp sometimes. I don’t know what’s going to happen when I go off in my room, a week after this film opens, to write the script. But I’m going to start writing.
Q: Do you have an outline?
A: Yeah, definitely. A really robust outline, which is pretty intricate. But now the standards for my outlines are higher. I need to know I’ve won already. I’m almost there but I’m not quite there.
Q: Have you been worried about the news of the ending spilling out?
A: Yeah. Actually, in fact, I didn’t put the ending on when we first test screened the movie. I wouldn’t do it, somewhat out of fear that it would get out. But on top of that, I wanted it to win as its own movie. You like Split as its own movie? Well, guess what? You weren’t even watching what you thought you were.
Q: You must be smiling a bit. This is a redefinition of the twist ending. This is something you’ve never done before.
A: No, that’s correct. But basically it’s similar to Unbreakable. Can you make an origins story that the audience doesn’t know is an origins story until the last image of the movie. In Split, you really thought you were watching a psychological thriller. And you were. But it’s actually a comic book movie. And when the moment comes, when you’re watching it with an audience, the place just goes insane. It’s pandemonium. It feels great.