I've written a theory for Mashburn previously so in this theory I'll just focus on some small details that would support Mashburn being RJ.
First, RJ is very careful not to leave any real clues as to his identity. He would not arrange to meet Jane because it would be too risky. This is why I believe that RJ meeting Jane was a coincidence. Mashburn just happened to be one of the team's suspects for the murder in Redline. Redline is full of clues/evidence for RJ being Mashburn but I'd like to focus on how this coincidental meeting changed RJ's relationship with Jane. A few episodes before, RJ has Rebecca kill Bosco and his team to cover up the evidence uncover when Carter Peak's body was found. RJ was forced to do that to keep himself safe instead of it being a directed mind game toward Jane like we see in later RJ episodes. Rebecca killing the team and telling Jane that RJ missed him just means that RJ wanted to use Rebecca to the fullest extent seeing as she would have to be killed after Jane caught on. At the end of that episode, I believe that RJ's first concern was his safety and playing with Jane was fun but not his top concern. Then comes the episode Redline where RJ/Mashburn meets Jane by coincidence and RJ experiences real risk as he talks with Jane and manages to slide past Jane's radar. This is what triggers the change in RJ and Jane's relationship. RJ finds that he enjoys the risk of really playing with Jane. That is why in the second season final, RJ takes the very risky action of following Jane, intervening to save Jane's life, allowing Jane to see him (albeit with a mask - but body shape & height), actually speaking to him and taunting him in person. Remember in Pilot, RJ was content to let Jane catch his copycats but in S2 final RJ wanted to play with Jane. The difference is that Mashburn/RJ had met Jane and acquired a taste for playing risky games.
Second, when Mashburn returns in Red Hot, he at first pretends not to remember Lisbon's name and Jane has to remind him with "Agent Lisbon." Later when he's stepping into the room with the bomb, he steps out again and suggests that he and Lisbon should get together, calling her Teresa. Mashburn/RJ would have tried to throw Jane off guard by looking like he'd forgotten Lisbon when RJ obviously would know a lot about Jane's team. I think that Mashburn/RJ somehow guessed that there was a bomb and had to quickly invent an excuse for stepping back outside. Mashburn is very familiar with bombs (starting making small ones in Reform School) as is RJ who presumably made the jacket bombs that were used in the first part of Strawberries and Cream (S3 final). Mashburn could have guessed Yuri would try something like that (which is why he took the opportunity to get Jane/Lisbon on the case), smelled or noticed something in the office that meant bomb, or read Yuri's anxiety about the bomb. What I'm saying is that RJ/Mashburn pretended not to know Lisbon but stopped that pretension as a way to get out of the room.
Third, Mashburn took the time to flirt with Lisbon and he even slept with her. RJ would have known how risky it was that he had actually met Jane so I believe that to throw any suspician off of himself, RJ/Mashburn developed a relationship with Lisbon in Red Hot and then ordered O'Laughlin to kill Hightower, Lisbon, and others in S3 final and tried to get Jane to kill Lisbon in S4 final. RJ is very clever. He is clever enough to throw suspicion off of himself by liking Lisbon as Mashburn and nearly getting her killed as RJ. That would throw off Jane's suspicion. Also note that Mashburn is the only suspect with the same power of seduction that RJ has shown in seducing women over to his side and inspiring trust.
Fourth, in Red Hot, Mashburn stops Else from stabbing Lisbon with a knife. After Lisbon arrests Else and leaves, Mashburn holds the knife naturally in his left hand. I think most people would put a knife down immediently but Mashburn naturally lets it rest in his left hand - most people think RJ is left handed. I think Mashburn is the only suspect that we've seen holding a knife.
These are just observations that support my theory which can be looked up on the website. Thoughts?
