My theory,RJ could be either

1.) Robert Kirkland's twin brother (Or)
2.) Patrick Jane himself.

Regarding the latter, there is only one reason for me to believe is

I dont think there are many people as clever as PJ who has got such trmendous skills in reading peoples mind. Even if Red John could have been intelligent than Jane, the other accomplices/acquiaintance of his are definitely not that clever, which is just a piece of cake for Jane to know more about Red John from  all his acquaintances he has caught so far.

If Jane could not find someone, with all the skills he possess, it should be someone out of the planet (Multiple Personality Disorder).
I know most of yal would disagree with me, but just trying to rule a possibility

Now my best guess to be RJ is Bob Kirkland's twin.
Possible Reasons

1. He is assumed dead(not found the body yet).
2. He is an alcholic who has attended many Rehabs/cults (possibly visualize).
3. The time the twin went missing coinsides with that of Visualie's farm killing(believed to be the 1st murder by RJ).
4. A  funny one, but still - Kirklands twin should have same voice as his brothers which is the only voice that matches RJ's voice.
5. Bob Kirkland knew his brothers acquaintance with RJ.
     If he would have found on his own, he should have known a lot more about RJ.
     It is possible that Kirkland's twin after the first killing could have thought him to be       a different/new himself and named the new himself Red John. After that he could         have told  Bob Kirkland that he met a guy Red John who he is going to follow.
6. Bret Stiles has inside knowledge of RJ and when he hinted PJ about it, he was so arrogant that he thinks himself superior to RJ and PJ.
    The only reason he thinks he is superior is because he should have been the one       who possibly could have brain wasshed,encouraged and motivated RJ into                 committing his first murder.                                                                                                   Since Stiles helped Kirkland's twin finding the RJ in him, he could have                         possibly been greatful to Stiles and thought him as his mentor too.                                This could be the reason for Stiles' arrogance about his knowledge on RJ.

Please let me know, if the above reasons seem persuasible and also if
there is any reason not to believe Bob Kirkland's twin to be "RJ"