I think Red John is either Walter Mashburn or Kristina Frye. I am hedging toward the former for the following reasons. He is smart, well educated and seeks thrills as normal life leaves him bored. He has the resources to do what Red John does, and significantly, unlike many men of his wealth and status, Walter is self deprecating to the point where he wants you to think he is just a bored, pampered playboy who likes playing games but is no real threat.

Despite this Mashburn is likeable and yet despicable. I think he is good for it. He is also enigmatic enough to be RJ. JJ Larouche and Bertram have potential to be followers of RJ but not RJ themselves. Brett Stiles gets off on controlling people not murdering them.

Kristina could be RJ as she is a little too goody two shoes for her job description and it never was quite explained what has happened/likely to have happened to her. In any case I hope RJ;s identity is done well and does not turn to be really crass  -like be someone like Lisbon.

RJ should be consistent and believable with what the story has told us so far while at the same time leaving room for surprise, but not shock.