I do not know who Red John is, but I have a few things to say. 

1) The writers would be insulting the viewers' intelligence by revealing PJ as RJ. 

2) That Susan Darcy lady must have a skeleton in her closet!

3) At the end of the most recent episode, (5 x 03) did something not seem off to you when Lisbon goes to play poker with FBI and finds her boss, CBI director (Bertram) there too??! That scene has a weird vibe to it and I can see RJ having something to do with that gathering. Remember, RJ has a friend in the FBI. But I can't say for sure that Bertram is our guy because he seems taller than the guy in the mask in season 2 finale and because I don't see a director like that doing the dirty job by himself. 

Do we really know that Red John is a man? Could he not be a woman, or maybe a man and a woman? Or maybe he is a small handful of people who gather together and play poker once in awhile...

Okay.. Why can't I post this theory?! maybe cuz it's true ;)

Maybe it's a little far- fetched, but what if PJ helps RJ, without
knowing? Like RJ (or someone who works for him) hypnotized Patrick. This
would be the reason why RJ knows what PJ is about to do (and why he is
one step ahead of him everytime). I know it may be a bit unlikely, but
some people believe RJ is Patrick himself, or Lisbon. Remember that
episode when a guy walks into the CBI with a dead girl, but he thought
he brought a bag of potatoes? I'm not saying one of those hypnose-
freaks is RJ, but maybe one of them works for him. Think about it..

I believe that we are being led down a garden path with all this talk about Red John being a character we are already familiar with.

I believe that Red John has been exceptionally honest with us.

I believe it is a man whom we have yet to meet.  Reginald is his first name,  Surname is Onn.
He is known as Reg Onn,  therefore he told us his name, he has effectively hidden in plain sight...

But then again,  I'm not sure I know what I'm talking about at the best of times...

according to myself Red John is Red John...I mean that he's an alterego of a person whose name isn't even John, but we haven't see him yet...and I think that even all the people who work for him know him, like O'Laughlin - too closer to Jane and stupid - 

I don't have any speculation to who Red John really is, but I doubt it is Patrick as Red John is face to face in a scene at the season 2 finale.  He is masked but that is Red John's voice so I doubt it can be Patrick even though that would be a huge twist!  Also he spoke with Patrick in the season 4 finale and ordered Lorelei to cut off two of his fingers... It is not logical but also, I think someone very close to Patrick is an associate to Red John that is why Red John always has the upper-hand because it would not suffice just having a member of the FBI as Red John's associate as they do not know and do not find out everything that is delivered about Red John or just the actions of the CBI in general... 

Red
John can be Max Winter (Jack Coleman), also called sometimes "Winters", the widower of the episode 3×04
pushing Jane to shoot at the range, and exact revenge on RJ,
strengthening his purpose, and then gives him his gun with which he
avenged his wife Jenny, who was killed in 2001 (is released because his
son shot but the killer and rapist was already dead, killed by another,
perhaps, and the prosecutor can not proceed). Jane with that gun kills
Timothy Carter believing that it is Red John.
Winter is a businessman
and manager working in construction and industry (agent real estate investment, as say him), it also brings him a
wedding ring as Jane, is rich and buy an old warehouse of evidence, and
the DNA is exchanged exonerate the murder of wife, Henry Dahl, to be
able to kill himself. Buy a gun from an arms dealer, then someone like
Manuel Montero, the friend and supplier of Todd Johnson, killed by
O’Laughlin to the museum on the orders of RJ to frame Hightower. Johnson
played with him in football, as O’Laughlin play football. O’Laughlin
knows Brother Steven Wench, the undercover FBI in Visualize, on which
Stiles knows everything. And Stiles also know a lot of RJ (and an adept
of the Wench’s group is Ron Deutsch to Visualize that steals the gun and
the cell Carter, then Wench can be a mole of RJ in FBI; or the mole is the FBI chief in person). Another person
involved in the construction companies as Winter, is Jack Tanner (1×03),
an worker alcoholic, father of a victim, Christine, kills in a construction site. It ‘the
same surname as Orville Tanner and his son Dumar. RJ has some knowledge
of building as built himself a hut on the farm Tanner in Sparrow Peak,
registered as owned by Cutiron properties, fictional company of Roy
Tagliaferro, as RJ solutions. RJ initially hides the body of Carter Peak
in a construction site of a shopping center. In a shopping mall (the same?) Jane meet Carter.
RJ pay an airplane pilot who specializes in writing in
the sky for the smile of the scene of the murder of Emma Plaskett; Max
and his son, Jackson, build model airplanes operating in the
workshop-garage. Max Winter recalls the name Max de Winter, the widower protagonist of “Rebecca”, the first wife of Hitchcock’s movie. The Las Vegas zone (with Debonar way, a word use by RJ) is a kind of construction site finished, foreclosed and abandoned, like the house where they find Kristina.RJ
calls the women victims’ his wives or brides, “writes Wagner (Jane is not
surprised, perhaps Linus, being expert modus operandi of Red John, he
used a term already used by RJ himself). Rebecca, the secretary of
Bosco, proclaimed himself the “girlfriend” or at least the lover (as
Rosalind) of RJ. Girlfriend living-dead wife, maybe so in the jargon of
RJ. Lorelei is also perhaps one of his lovers, as well as Jane.
PJ
causes RJ on TV. Jane and Winter are on TV by Karen Cross, the former
prosecutor who did condemn Dahl, and who has followed the case Winter
(which has already gone on television to confront the mother of Dahl,
pretending to be forgiven through faith in reconciliation, something
similar to the words of Carter and Lorelei) and Jane does the trick of
fire with the candles (which by the way also O’Laughlin says he can do,
maybe that’s how he set fire to Johnson). Panzer
causes, instigated by Jane, RJ just to show Karen Cross. Maybe RJ is a
faithful viewer of this program, since it kills a few hours later, then
the transmission is supposed to have seen the same evening? If Max had
something to hide, maybe Dahl pushed himself to kill his wife for him.
Maybe his wife suspect of him (as Todd's girlfriend). And Max is one of the few people that Jane
has a hard time to “cold reading.” If we want, we can find many other links. And Bertram (remember the newspaper of Carter and other clues, as the poem?) can be the mole in CBI. Or is CBI Ron? Bertram go to prosecutor's office to drop the charges against Winter.
The RJ plan is manipule PJ, and is start with the "Tyger Tyger", goes on with Johnson case (the cop's murders is maybe only a bait to attract CBI) and Carter: the gun of Max is important in this "big picture", for transform Jane in a RJ's follower.
Then, the message He is Mar; could be
He is Max, or He is manager: maybe written by the hooker, who didn'y know well english language, on the advice of renfrew, before dying both.
Will be just coincidence or false clues? As Jane says: "There are no coincidences".