I would bet on a character who has not been seen yet in the movie. I remember very well the appearance of Bradley Whitford in Strawberries and Cream: Part 2 (S3, Ep24), when he claimed to be Red John. Timothy Carter was so calm, enigmatic, with such a creepy charm as Anthony Hopkins after brain surgery Ray Liotta. I never liked Bradley Whitford, nor his films or roles, but at that moment I thought: "boy, this is one real Red John". Honestly I'd be happy if Red John looked, talked and reacted that way. For a brief moment I felt that this character perfectly fits the villain Red John. Ultimately, an actor who appeared in the film for the very first time, successfully recreated the character of the man, ruined the life of Patrick Jane and I felt it plausible.

The implantation of Red John in an existing character would lead to much confusion in already past events and cause questions that can not be answered. Eventually I would be quite disappointed if the ever smiling, sweet and charming Patrick Jane suffers from a twisted disease, turning him into the supreme evil.