OK so, not completely convinced Red John is Ron, but I am convinced it isn't anyone involved in "Tyger Tyger" .. I do however think Red John has something to do with the Tyger Tyger group.  The William Blake poem, The Tyger, is first quoted by Red John, so the
claim that he has nothing to do with it cannot be true. I believe Red
John is the "Tyger" to Patricks "Lamb" ("Did he who made the Lamb make
thee?")
I am convinced, based on religious imagery in the show, that Red John sees himself as Satan, or Satan- like figure. This is reenforced by the recurring references to William Blake, and why the next episode is called "The Great Red Dragon." (also when TL enters the house where BP is killed, 666 and a pentagram can be seen on the wall) Other religious symbolism throughout the show lends credence to the theory as well. Lorelei Martin, for example, seduces and tempts PJ with money, security, etc on behalf of RJ in exchange for the head of Theresa Lisbon, a little like Eve and/or Salome.

I think its likely that RJ possesses the same talents that PJ does for hypnotism and suggestion, and uses this to control certain members of the police and others (the "Tyger, Tyger" group etc).  He may see himself as a Satan commanding his own group of fallen angels. The police that are not involved, or the innocent members of Patricks team can be seen as the "good" angels in that analogy. (Interesting to note here that Patrick's wife's name was Angela).

Why Ron though? I think he makes sense, he has been present since the first season, he definitely has given himself a Satan-like haircut. He does clerical work, which is something PJ said of RJ in an extra scene of the pilot episode. He is involved with the CBI so that he can keep an eye on the progression of the case, as well as surround himself with death etc, but does not have an active roll in solving the cases. I like the idea of him being the "Where's Waldo" character that everyone mentions, and I like that his name is close to "Red John". We know that RJ likes to keep his alias' similar. He matches Rosalind Harker's description, and does not go against the rough picture we get from Sophie Miller.

A few problems with Ron as a candidate: I am a little troubled that the character hasn't been given a last name, perhaps he is too small of a character to be a good choice for Red John and that Rosalind did not mention that RJ had facial hair.