Red John is like Waldo, on background, as says Bruno Heller, and we have already seen him...so my theory is..
1.red john is not somebody from the main cast(unless we find proof that one of them was in the backgroung when something important happened)highly unlikely
2.there's way too many suspects that are way too obvious.
3.it has to be somebody who had a really small role in an episode and shook hands with patrick maybe once and never saw him again...only to be seen in the background that is if we pay attencion...maybe somebody showing his backside during a scene.
4.i used to think cbi ron was the perfect suspect....but now i'm beginning to think he's way too obvious.