Well, my thoughts are that Jane/jisbon has yet to decode the blake society list, and thus I am in favor of a Rigsby slasher ending to this lost television show. 
Anyone find it bizzare that Rigsby instantly knew where Jane buried his wife. The "Red John" S6 has Rigsby in the back of the truck stating clearly and without hesitation the exact name of the cemetary where Jane buried his wife. This is one strange thing among many strange things Rigsby does while in the back of the truck... calmly sitting the back of the truck. 
The only thing that makes me think that RZigsby is not RJ is the the blind lady says that RJ is under 6 foot tall. Rigsby is clearly over 6 feet. However, I do understand that RJ had many helpers. 
Oh Wait... Lorilie Martins also hits Rigsby over the head when she is going to kill her victim. Would Martins hit her beloved RJ over the head??? Methinks, not!
Aside from those two problems, Rigsby could easily be RJ.