Prehaps it is a couple of the original seven suspects. In season 6 episode 4, Reede Smith reveals that Bob Kirkland's theory of there being an organisation with law enforcement agencies is indeed correct. With this, it is plausible that this organisation is actually called Red John, and that there is no one serial killer. Possibly, instead, having a scenario like having someone charming (like Stiles) to entice people in, Bertram to run the organisation, and have people like Haffner or Smith do the killing.

Either that or the organisation is going to be something PJ and the team are going to expose next season, when there is no more Red John to deal with.