Film Theory: Split Is More Than A Standalone Sequel To Unbreakable...
3. How Unbreakable Fits In
As for the link to Unbreakable then, it’s pretty openly signposted at the end of Split that Glass will be the uniting force between the two movies. Bruce Willis reprises his role as David Dunn, the impossibly strong superhero character of the 2000 movie with powers to predict the future. Samuel L. Jackson’s Mr Glass is mentioned only by name, but it’s enough to confirm that the world of Split and Unbreakable is indeed the same one.
One problem with The Sixth Sense becoming canon in the Shyamalanverse that has been brought up by fans is that Bruce Willis already holds two parts, as both child psychologist Crowe and good guy David Dunn. Honestly, this has an easy answer. Of course, he could just be ignored as two very similar looking characters in the franchise and have Willis playing both - but more realistically, it also has to be noted that Malcolm Crowe is dead. He passed on in peace at the end of the film, resulting in very little issue with the actor holding two roles in the same universe.
If anyone was going to drop a bomb that all their films were connected all along, this director is your guy, and there’s moments in plenty of his movies that could be applied to the events of both Split and Unbreakable and the logic that they share.