Okay, first of all, there are many elements that the producers seems to put forward to make sure that the public will notice so I'm guessing that these people are people of interest but not Red John himself.
First of all, Agent Ray Haffner is suspicious when he leaves the room when Teresa Lisbon asks him if he worked on the farm where Red John first drew the smiley face. He has shaken Jane's hand however, he wasn't at all smart enough to stop Jane's plan to reinstate his old team in the early season 4.
Then, there is Bob Kirkland. In my opinion, he is a person to look out for, although many people are saying that Lorelei Martins was behind bars when they shook hands and that there was no way for her to know they've shaken hands, there are always ways for people to know things in prison, especially if you are connected to Red John. He was the one who asked Alexa Schultz to ask Minelli to keep the FBI in the loop for updates on the Red John case now that Jane joined the CBI and Red John knows that Jane is a very smart man who could make a break in the case.
In the Bosco murder case, at the end, Rebecca told Jane that Red John had Bosco's team murdered because he knew Jane wanted the case back, so maybe Judge Manchester is somehow involved because he probably knew that Jane was going to pull some kind of trick to annoy the FBI and don't forget the "He is Mar" written on the wall where the "r" could well be an unfinished n.
Maybe Red John might also have a link to Jane's past considering the whereabouts of Lorelei Martins' body and our knowledge of Jane's past.
Anyways, just thought I might explain a bit of what I think
