I made some observations, some of which are old and some of which are new. They make me feel that Red John is Haffner.

1. If you see the scene where Rosalind calls Jane and says "Roy is here", RJ sits with tea cup in his left hand. Look at the way he sits. Now compare this with the way Haffner sits when he visits Lisbon in her office. He sits exactly the same way. The camera will show you that for just a few seconds, when he is talking we see only his upper body. I think this could be a vital clue

2. He is a surveillance expert. He can watch people using all his resources, that is why he is ahead of Jane.

3. He was 21 and was in Elliston Farm when he was a member of visualize. He tells Lisbon that he doesn't believe that Brett Stiles is God, but happens to believe in somethings he says. All other members of Visualize believe Brett Stiles to be God, only RJ can say like this.

4. The way he walks in the hospital in Season 6 Episode 2 is exactly the same way Red John walks in Season 2 Episode finale, where he kills RJ copycats.

5. Well built, not too soft or too strong, his hands look exactly like Roy, medium aged man, height is just below 6, short hair, good looking. All this fits him.