Blatty’s ELSEWHERE novella (found in the 999 anthology) may be the closest we’ll get to a sequel; it features some priests in cameos, though neither are Karras or Merrin, and they’re fairly inconsequential to the rest of the story if memory serves.
But apparently there’s something called Demeter, a confirmed (I think) sequel, which he’d been working on for years. Anyone know what happened to that?
M.I.K.E.