I know this is going to sound mental because he died years ago. But do you guys think Bosco could have faked his death somehow? :P
I listened to his voice all day yesterday and he literally HAS Red John's voice. Not just similar but 100% the same. 
I know it's crazy, I even made fun of people who thought it was him before..I apologize to them now.   

Reasons it could be him: 

-He loved Theresa so wouldn't hurt her
                                     
-He told PJ when he was dying that he will come back to haunt him (it could be like a sick joke)
                                     
-Surprise element because I'm quite sad that they let us see the sheriff in the promo, I can't believe they would do that..I will be furious if he is RJ and we found out from the promo a week before the Red John episode.

-Voice matches a 100%
-His secretary Rebecca seemed to love him, maybe they worked together.
-Lisbon had feelings for him in the past so he must have a side that makes him attractive to women.  

Reasons it can't be him:

-The doctors at the hospital probably wouldn't lie about his condition, he was dying
-Rebecca went to visit him with a gun so it really looked like she wanted to shoot him
-He has no hair and he is not thin

Anyway, it is probably not him and he is really dead. But at this point I would accept anyone else but the sheriff. They've really ruined it for me with the promo..:(