Red John is NOT Brett Partridge! It's too obvious. The writers have given themselves a lot of room to define who Red John is and they can choose anyone from Jane's list of suspects, anyone on the CBI team, and they can choose someone new.
If the what the writers are trying to do is create a surprising and enjoyable ending to the series, they won't choose Brett Partridge, because he is probably the most obvious "bad guy" available. He's a misdirect (probably an homage to Psych's Mary Lightly, who was also creepy but innocent). That's why it's unlikely that the writers will choose him when they decide who to make Red John.
The identity of Red John will probably depend on how "artististic" the writers of the show think they are. If they fancy themselves Hitchcocks, then Red John will be either Patrick Jane, Lisbon, Van Pelt, Rigsby or Cho, just because they would want to surprise and shock the audience.
If they suck at their job they'll choose one of Jane's suspects from his list. One thing that would be essential to the finale would be Jane's sense of shock at who Red John turns out to be, and you can't have that if he turns out to be on Jane's list. So, assuming that Red John isn't on Jane's list, then that leaves the CBI team, a lot of females (who he said was unlikely, which makes it dramatically more likely for the writers of the show to choose), and lastly someone we haven't met yet.
So my proposal is that Red John is either a CBI team member (Patrick Jane, Lisbon, Van Pelt, Rigsby or Cho), another female on the show (like the CBI press director), or a completely new person.
