A lot of theories on here have been a reach. I mean 75% of them make no sense. The list from Heller is legit and Red John is on that list. Brett P. is DEAD and not coming back. That leaves us with 6 suspects. The lead up for season is going to keep everyone guessing on who Red John really is. It seems like with the conclusion of each episode from season 6, everyone is changing their theory. Now everyone believes it is the Sheriff.

I like how Heller as a writer, is keeping everyone guessing and on their toes. This reminds me of season 2 of Dexter where Dexter is being investigated as the Bay Harbor Butcher and he has his colleagues conflicting with one another and Agent Lundy is sitting back realizing that the Butcher is having the cops running around mindlessly. That is what Heller is doing with us. 

As a writer, Heller needs to bring the conclusion of this story arc to a neat and clean. Additionally, there are characters on the list that could be used later in the season such as Bertchum and Styles. So it remains to be seen who Red John will turn out to be.