I believe Red John has been introduced to the viewing audience in another identity. 
I  decided to look at this as more of a casting issue.
The actor they choose for this character must have "acting chops".

Most likley:
Male
Mid to late 30's
Caucasian
Around 6 ft
Member of visualize.
Has shaken hands with Jane.

 He is not often selected on these type of sites, which is odd because he is one of the few characters that meet all of the requirements(that isnt dead yet). 
I believe they have selected Agent Ray Haffner, played by REeD Diamond, a Julliard trained actor.