I make it to be Patrick himself. He does not know it tho, Red john is the stronger side of his personality split or maybe they might go for possession giving Patrick the gift he thinks no one can have.
The red john sign on the walls kinda looks like Patrick with the slanted eyes and the top of the circle creates goes off in the same direction as his.
Also this shows how he is always one step ahead and can read patricks mind.
Red johns words at the end of s5, I will kill until you catch me or I catch you. Is one and the same thing.