While I'd like to know what happens to Katie I don't know it's necessary. The demon was using Katie to get to Hunter after all.
How would one pursue finding out what happens to evil Katie post PA2? How does one continue the voyeur aspect of the series doing that?
I see two options.
1-The demon comes after 14/15yr old Ally for whatever reason, cause he/it killed everyone else and just wants to have a clean slate. Ally, traumatized by coming home to mom&day murdered is in a psych ward. Complete of course with cameras. This could be where possessed Katie comes into play.
2-The story for PA3 features an all new cast encountering new/different paranormal activity.