One of the biggest mysteries of The Exorcist: Believer is how the MacNeils will be figuring in. Ellen Burstyn's Chris will appear, helping out families when two teens get possessed in this requel. However, the trailer hasn't confirmed what will be Regan's position, or if Linda Blair will indeed be reprising her role, per rumors.
Source:https://www.cbr.com/the-exorcist-tv-series-finished-regan-story/