So, this is really offbeat, and totally not plausable,but, everyone I talk to thought it was cute. Here is my theory.

Red John is actually tea. Yes you read right, tea. Here's why I came to this conclusion.

1... It has been said that Red john has been there from the beginning. Maybe not always in plain sight, but as a Where's Waldoesque element. Tea is always there, wether PJ is drinking it, or it is in a CBI ofifices or in a victim or suspects home etc....But tea has been there from the start.
AND
2.. PJ has shaken hands with Red John..We have seen PJ shake hands with tea on more than one occasion. He can't help it..tea aka Red John are his biggest weakness. PJ realises in The great Red Dragon that tea MUST be Red john when it is knocked out of his hand and the cup shatters.

Hope this makes you all chuckle, can't wait till next week when we finally get our answer!