i have a few theories who it may be
Patrick's Dad
Van Pelt
or Patrick himself :((((((((((((((((((((

first of all in season 1 there is an episode in which jane has flashbacks of his dad, his dad is a conman. it is stated that jane is much like red john, father and sons are alike? also it is clear janes dad probably did resent him as he wanted to make a success of the skills jane has but as you see in the flashbacks jane doesn't want to do what his dad wants him to do. Perhaps the only way his dad could feel powerful is by becoming this serial killer, now he is equal to jane and has had revenge.

Van Pelt is not red john but i think she could be his daughter or something (obviously then i doubt jane is red john if she is linked):
1) you never know much about her throughout the show
2) she believes in psychics so maybe she believes in red john, who is much like a powerful psychic or something
3) red john would have to have some computer knowledge, van pelt is the one always in control of the computer, so she clearly has this knowledge, when there has been messages sent through she probably knows how to do that.
4)She is the new one, perhaps she wanted to join to gain inside knowledge on jane, also she stopped relationship with Rigsby perhaps if she was closer that what mean risking being found out.
 hmmmmmmmmmmmmmmmmmm

finally we have Patrick Jane
I REALLY REALLY REALLY HOPE IT'S NOT HIM I WILL BE SO UPSET
I'll keep this short but i guess he may have a split personality as jane does share a lot of traits similar to red john.
or perhaps he was once the red john but someone else has now taken over?
i'm not really sure and i try to not think it will be Jane but i really feel like it will be linked with him in someway.