Apparently Split Gave Away Its Twist Ending On Its Poster
A rather astute user on Imgur noticed the fact that if you take one of the international poster for Split and mapped it up next to the poster for Unbreakable, you'll see that the glass is cracked on both posters in such a way that they connect. So instead of two panes of glass being broken, you see one gigantic piece fractured over the course of two images. Kevin has literally fractured David's world in this visual metaphor, which is interesting considering where M. Night Shyamalan is going with these movies.
Considering that there could be a third Unbreakable universe film on the way, the fractured glass indicates a sort of disruption in this universe's fabric. What's causing this disruption? The existence of superpowered/special ability wielding humans, such as David, Kevin, and to the alternative extent, Mr. Glass. With the discovery of such persons, the world will start to change in order to combat any threats these people may pose to the rest of the world. Think of this as M. Night Shyamalan's version of X-Men, except quite possibly being set totally in Philadelphia and on a more intimate scale.
Any sort of follow up to Split is surely being discussed at Universal as we speak, as the film has managed to bring in a little over $100 million on a $9 million budget. With two solid weekend at the box office and many critics in its corner, it looks like Shyamalan may have a bite at the franchiser's apple yet again. Only this time, the series on the line will be from his own mind rather than an adaptation of a beloved animated series. Whether he sticks the landing on Unbreakable 3 is yet to be seen. But what we can see is that much like his movies, Shyamalan's marketing campaigns are sometimes hiding the truth right under our noses.
Split is currently in theaters, in case you haven't seen it or need to go over the clues again.