Sheriff McAllister, in his one episode, nearly abducts Van Pelt and seems very threatening and capable of being a serial killer.  If I had time (and knew CA geography better) I would go back and figure out where the Red John murders were in relation to where McAllister is sheriff. 

As for all the discussion of Red John having great contacts and influence, I believe this is a misdirect.  Red John was just a smart serial killer until he killed PJ's family, then be seems to have become more and more influential.  I believe that Brett Stiles is using Red John to get at PJ.  Brett is a great need to control others, and to use them to his own ends.  Ever since PJ came across Brett's path, Brett has desired to control PJ.  Brett is using all his influence, sometimes to Red John's benefit, sometimes to his own benefit, to gain control of PJ.  This is a Moby Dick story where PJ and Brett are both Ahab and the whale.

The other characters who seem to be part of Red John's minion (he is many) are actually being manipulated by Brett either directly or indirectly to serve Brett's purposes directly, and sometimes Red John's indirectly. 

This theory will work with any of the lesser characters being Red John and Brett pulling the strings, so Partridge could also be Red John in this scenario, however, I like picking McAllister because he is the least likely suspect.