In the last episode, Fire and Brimstone, Grace goes to visit Brett Styles. And he is talking about a man from Ecuador that he helped. He calls him "ambitious" and says he is the minister of state. Then he quickly rushes Grace out.
Is this some sort of clue?
I'm not sure who it is, but assume I will be disappointed. I wonder if it is possible that Jane already knows who Red John already is. Jane always knows who the murderer is way before anyone else (most of the time at the beginning of each episode he already knows). Anyway, it is possible that Jane just wants RJ to feel comfortable so he can make a move that will hurt RJ to the same degree that Jane himself was hurt. In the case of RJ, I think the way he could be hurt the most would be to destroy his reputation and embarrass him beyond any repair. To humiliate "the great" RJ would be the ultimate revenge.