I have no clue who they will pick to be Red John, but there are some people who I would HATE to see as Red John, and would be terrible for the show, in my opinion:

The "Already been done before" characters:
-Jane's dad:  A little too "Star Wars" for me, having the bad guy turn out to be the good guys dad...I also think that they would have to do too much extra explaining if it was Jane's dad to tell us how Jane wouldn't have known when he was a kid that his dad was RJ

-Brett Partridge:  "Dexter" already has claim to the CSI dude being a serial killer...I wouldn't like to have Brett be the same thing...Obviously, you could say there are plenty of differences, which there are, but still too similar for me.

-Patrick Jane:  Would be way too "Fight Club"-ish if it turned out Jane was doing all of this in his subconscious...plus i think it would just kind of ruin the show if it was Jane.  Everybody loves Jane and would hate to see him as the bad guy (I think).  Also, I don't think it's using much imagination on Heller's part if he chose Jane to be RJ (just look at how many people have already come up with that theory!!  #1 suspect on this site!!!)  Can't be Jane, too many people have guessed that, and way too many other reasons for it not to be him!

Others I would hate to see as RJ:

-ANY female character:  ESPECIALLY Rosalind.  You can't just tell the audience a million times that RJ is a man, then turn around and make it a woman...and if it's a BLIND woman on top of that, I would hate the show forever.  No way could RJ be a blind woman, that's just dumb!

-RJ as a "group of people":  RJ needs to be a singular man.  Obviously, he has plenty of people who work for him, but there NEEDS to be a single RJ to be caught and killed by Jane.

-Laroche:  if RJ is a 300lb man with a sissy little dog, I'll be a little upset!  haha

-ANY dead person:  If they try and bring of the "dead" people back to life to be RJ, that will be really lame...

That's all I can think of right now...It's a shame that Bret Stiles is too obvious of a choice, because I think he would be a perfect fit for Red John.  
If we are supposed to be "shocked" at who RJ is, then it can't be Jane, Brett, or Bret, because those are the top 3 suspects from us, so that would be kind of dumb