Not only did William Peter Blatty write the second sequel to this film, he directed it as well
Yet Blatty did get around to writing a proper sequel: 1983's 
LEGION... [DOC] [TLE] William Friedkin Directing William Peter Blatty's Exorcist Remake ... [PAR] Nov 15, 2009 ... William Friedkin Directing William Peter Blatty's Exorcist Remake?