I haven't seen Split yet and though I haven't read the full article, I read enough now to pretty much know the gist of the twist ending. Damn me and my curiosity. So, according to said article, Split is directly connected to the Unbreakable universe. M. Night is going forward it seems with Unbreakable 2 and the two films will (or may) be connected. Also, It seems Bruce Willis is on board.
http://www.hollywoodreporter.com/heat-vision/split-movie-ending-explained-unbreakable-2-bruce-willis-planned-966669
http://www.hollywoodreporter.com/heat-vision/split-movie-ending-explained-unbreakable-2-bruce-willis-planned-966669