I think it is setting up to be Bertram because he is so interested in the case and his voice could easily disguise as Red John and he would have been next Boss after the whole limo fiasco with Wainwright. Or it could be Ray because Red John didn't kill Lisbon because he "liked her" (Jane said) and Ray visited Lisbon in hospital and said "I like you Lisbon. You and me, we'll have our time." Or if Partridge really isn't dead, it could be him because he's creepy and all plus his voice sounds calm and weird like Red John. How else would he know to say "Tiger Tiger"? If Red John told him to tell Lisbon that, I don't think he would waste his dying moments on that. And what better way to hide your true identity than to make Jane think your real identity is already dead?