What was the original plan for the new Exorcist trilogy?

We first learned Green was working on a new Exorcist movie in late 2020. Plans for a trilogy were announced in July 2021—well before the October 2022 release of Green’s disappointing Halloween trilogy-capper, Halloween Ends—with the added excitement that Ellen Burstyn, who played Chris MacNeil in 1973’s The Exorcist, would be returning to the franchise. The films were touted as being like Green’s Halloween trilogy—picking up the story directly from the original film, and ignoring all the existing sequels.
In July 2023, we learned the sequel to Believer would be called Deceiver, and that it was slated to arrive April 18, 2025. (As noted above, that’s no longer the case.) There’s no title or release window for any third film as of yet.