Red John is NOT (1) a woman (ii) Patrick Jane.  Red John is (i) someone with resources, charm, and connections (ii) someone Jane has met previous to season five, episode 8. (iii) extremely smart 

Assuming it's not some random rich dude from a past case, Walter Mashburn, Bret Stiles, and Gale Bertram seem to be the only people with the necessary connections, resources, charm, and mental capacity.  

I just can't picture Gale Bertram having the charm to manipulate people the way Red John does. It also seems rather cliched. The head of the CBI is Red John. I think Bertram is likely one of Red John's people, but not actually the man himself.  

Bret Stiles just seems too obvious. He is also too upfront. He openly manipulates people. He openly runs a powerful organization. Why sneak around killing people? Why moonlight as the mysterious Red John? It doesn't fit with his character. 

That leaves Walter Mashburn. He also seems a bit obvious. However, he has the charm, the connections, the resources, and the motive (boredom). 
There just doesn't seem to be anyone else who could plausibly be Red John.