As discussed in another article from my interview with M. Night Shyamalan, it’s something that the writer/director definitely wants to do, but right now it’s merely a concept rather than a fully-fleshed out idea.
Split is the first sequel he’s had the opportunity to make, and it certainly doesn’t look like it will be his last either.
Because of the Unbreakable tease featured at the end of the movie, there is every reason to believe that we will eventually get to see Bruce Willis’ David Dunn face off with James McAvoy’s Kevin Wendell Crumb.