I don't believe that Red John is one person, but really several people all controlled by Kristina Frye, the real "Red John". She is using hypnosis, which is very prevalent in the show, to have others commit the red john acts.

-She has been involved since Season 1.
-She is the only one I believe that could be so omnipresent and omnipotent. No one on the list could have possibly come up with the same list, unless of course the producers don't care about realism and just go for shock value. But with Kristina, who believes/faking that she is a psychic, I would believe that she could. She is very much the same as Jane and could produce this.
-She also disappeared during Season 2 finale and Red John conveniently shows up.
-She has not been present in the show for a while, so this would be a major twist.
-This situation where she controls others would also allow the producers to have another fake Red John and thus prolong the mystery. Face it, once revealed  what will the show focus on and how impactful will following seasons be? If they catch Red John and then while Patrick is interrogating or just before dying, realizes that he is hypnotized  it will give Patrick motive to continue the hunt while maybe everyone else is satisfied of the end of Red John.

She is the only one I will not be disappointed with.

Also, I think the producers don't know and are reading this site looking for the best answer. ;)