Walter Mashburn - Fits all the given clues, while rich and able to exert influence he would have limitations where money isn't the soul power, this would explain how Bret Stiles could have info about his doings and feel now fear of giving them out as he would be an equal and possibly bigger organization. My view is that I see alot of good writing possibilities around him being RJ and I think he is a developed enough character to fill the role but not be disappointing as RJ. 

Bret Stiles - as stated above he seems to have indicated that He is aware and knowledgeable of RJ and his organization but that Jane and RJ game is small time and a bit childish, this could be to throw jane off his scent but with Stiles also being Older and I don't believe he and Jane have been seen shacking hands when they met.
I think from a writing standpoint if Stiles is to be RJ then RJ would be an organization with multiple people doing the deeds of RJ, I think this could be well written but could also be a bit of a disappointment.
CBI Ron/Background Character - While I have no evidence that they are not RJ from a writing stand point I would hope that we aren't given RJ as out of the blue, I'm very much expect and hope that when Bruno Heller compares RJ to Moriarty it infers that we will know RJ as a character in some fashion and not just a face. 

Any of the Women on the show - RJ is consistently by those who speek about him as a man, and RJ seems to have a way with women and an ability to sway them, I just don't see this as possibility. I don't see how to write away for any of them that seems overly plausible or that won't disappoint. 

Brett Partridge-I get the since that RJ had his eye on Jane from the very begining (im basing this on the sceen with Alexa Shultz talking to Kirkland in the limo at the start of Janes carrer.) When Jane meets Partridge the do not like each other, do not shake hands, and I feel like RJ would never be so brash and unlikeable even for appearance sake. Plus It would just be disappointing if thats how the write it, even with filler information about the character.

Kirkland - I say no, strictly because too obvious and he met jane after Loreli was in Jail so she would have know they had met. 

bertrem/laroche/minelli - Mostly I put these as not viable because of the way they have been portrayed in the show, while they have a few clues to support them, they have all worked closely with jane and seem to be able to be manipulated by jane. Plus I just frankly would be disappointed to see them as RJ.

After Thoughts - Scenes that make me why?

Whats with the Poker game, it seems way to prominent and a bit unnecessary even for the episode where Lisbon used her connection to get a warrant. Plus Bertrim seemed to be taken aback when Lisbon was there.

Of course the Limo scene with Kirkland and Schultz, it seems way to obviously a clue and like they are just flat out telling you they have some nefarious agenda. I will say that it bumped up the idea that maybe there are more than one secret organizations at work here or having a power struggle and Jane is seen as a way to resolve the conflict.