My theory will probably not be among the most popular theories here but here is how I see the things.
We all are aware that only people from the list of 7 were able to be on the right places in the right time for killings that Red John committed.
We are also aware that Heller already stated that "we can trust the list," but also that we would be disappointed when we find out who Red John is. We also heard that after some time in season 6, it will no longer be about who Red John is, but about catching him.
Therefore, I can state my theory here:

Red John doesn't exist as a single person. There might be a leader of the Tiger,tiger organization, but that's all what his job is. All of the Tiger, tiger members are actually Red Johns for themselves. The organization kills everyone who comes in their way somehow, and acts under the alias of Red John. It's simple, they have many regions covered, they act together, and share their assignments.

Sheriff Thomas McAllister is, for example, Red John for Napa county, Gale Bertram's job is to keep Patrick Jane in his sight, and under his control. We've seen Reede Smith in action in the last episode. Brett Partridge's job was removing the existence of  eventual evidence that was left on crime scene. Bret Stiles' job... probably the biggest one, he's most probably the brain behind this whole organization. Ray Haffner... well, you can call him Roy Tagliaferro if you preffer.
The thing is that the list is probably too small for everyone that is involved.

Also, remember the "He is man" clue from Mexico, in an opening season. Thinking about it now, "he is many" would fit perfectly.

I've kept it short and brief, so I probably left some things unexplained.