It's not a sequel or a prequel or a reboot or a remake, but a film based on Mark Opsasnick's book The Real Story Behind The Exorcist is on the cards from Vertigo Entertainment (The Ring, Quarantine). 

The Exorcist story, The Haunted Boy, is a single chapter (serialised in full here) in Opsasnick's tome of supernatural investigations, subtitled A Study of The Haunted Boy and Other True Life Horror Legends. In it, the author tracks down and speaks to the priests involved in the 1940s exorcism, local residents from the time, and the actual "Rob Doe" who became Regan in the novel and William Friedkin's film. Hmm: sounds like a similar approach to Requiem, the German film that told a close-to-the-truth version of the story of the exorcism and death of Anneliese Michel, which also inspired the Hollywoodised The Exorcism of Emily Rose.

It sounds like this might make for a decent Angel Heart type mystery, although the screenwriters will be hard-pressed to squeeze drama out of the essay's anticlimactic conclusion.

But whatever transpires, it can hardly be worse than John Boorman's hapless farrago Exorcist 2: The Heretic, and there should be no need to film it twice.