1. Ruling out any possibility that PJ himself is RJ. Why? That's just odd writing on Heller's part if it turns out to be some Fight Club scenario. Plus, there are so many evidence pointing AGAINST that theory.) 

2.Ruling out any NEW suspects that are featured at the latest seasons both 4 and 5. ESPECIALLY KIRKLAND-freaky homeland security guy. Why? I know his voice matches and everything but HE IS A FAR TOO OBVIOUS CHOICE. His character is an intriguing one, and could probably be RJ's friend but RJ himself? I don't think so. In fact, any new suspicious characters introduced to the series from S4 onwards I believe are either on RJ side or PJ. THAT DOES NOT MEAN THEY are RJ HIMSELF specifically. 

I believe the writers' themselves already have slipped in that RJ character into the first 3 seasons of the series and in that way, when the final reveal occurs, it is revealing and make sense to the longterm viewers. Good writers often do that at least. 

3. Ruling out any one from the Team, except maybe Van Pelt might just fall into RJ side accidentally but even then, I doubt so. 

4. Ruling out females. Erm Rosalind, Kristina, Lorelai, Hightower etc.. I caught the writers saying RJ is a man in some random interviews so... and if not how can Lorelai be RJ mistress? Lesbian? No.BTW Kristina is an interesting character as well, I hope they bring her back in the sense that she in her state of mental instability contributes in some climatic scene haha such as helping RJ unintentionally. Also, I believe Lorelai might just flip and help PJ in the end. 

This leaves us with whoever we already know so far. RJ is a male, someone with connnections thus a person of authority and even more specifically shook hands with PJ before, someone PJ knows-from the older episodes.

Somehow, my strongest guess is Gale Bertram but LaRoche is a close second followed by Minelli . Gale Bertram fufils the criteria above and more importantly has been behaving the least suspiciously thus making him the prime suspect. I mean at the end of the O'Laughlin episode, he was at the scene of the crime. When someone supposedly took off with the fake RJ's gun. Coincidence? Hmm..also at the end of one of the recent episodes(the newscaster one) there was a zooming out effect when Kirkland was talking to Lisbon and PJ warning them against the rich evil guy? A picture of Bertram (i think?) was hanging behind in the background as the camera fades out and was kinda creepy in a Big Brother is watching you kind of way. I think it was intentional. 

I think, the writers themselves have no idea who Red John is and they are just making it up as they go along....perhaps using internet theories to steer toward a finaly reveal...

That would be so ironic if true :-p

Red John is very powerful, has a lot of disciples.Also we know that is a tall character.And in some way has some sort of style, like PJ.
Of course is absolutley smart.I think B.H is playing with us, showin a character in the shadows that is the mos likely to be RedJohn in the "Red dawn" episode ending.
And showing him again in the last episode, makes me think that maybe we are finally meeting RedJohn himself.

It is tall, intelligent, in a position of power(NSA), he has about the same age as PJ.He is kind of threatening, but not disturbing crazy.About the voice we could say that is like the other suspects, but he has something extra, call it the X factor.
What can I say?: ROBERT KIRKLAND IS RED JOHN.
Untill we watch him murder someone (most likely from CBI) this will be a theory.