After seeing episode 3 of season 6, where Thomas McAllister came out of several seasons' hibernation, I'm starting to think he could be the guy. Reasons why:

1) Bringing him out early in Season 6 like this puts him right back in the running.  Why waste an episode reviving an old character for no reason?
2) Lorelei Martins told Patrick something to the effect that "You and Red John are just alike. I don't know why you didn't become best friends the moment you shook hands" -- Another poster reminded me of the scene where Patrick and McAllister play rock, paper, scissors and PJ wins every time.  Get it, shaking hands -- that's how you play rock, paper, scissors.  The two of them had a lot of fun in that scene, so in a way they were just alike.  And the way PJ won every time showed there was some sort of mind meld going on there.  Kind of a tiger, tiger thing with the symmetry, etc.
3) In season 6 episode 3, McAllister did seem to "like" Patrick in a way -- as Stiles said, Red John wants to be friends with PJ. Red John has rescued Patrick once before in an earlier season, so the rooftop rescue seems very Red John-ish.
4) Also recall McAllister's creepy woodland encounter with Grace, who now seems to be a sympathizer (at a minimum) to Visualize.

So -- all this adds up to McAllister being a very viable possibility.  But far from a slam dunk.