The last few episodes have introduced, or reintroduced, several suspects: Oscar Ardiles was called to Lisbon's office, Ray Hafner offers Lisbon a job, and CBI Ron is the first to congratulate Lisbon on her 10th anniversary.  

However, one of my favorite theories is that Red John is Ellis Mars.  He was in the episode where the suspect is set on fire but still tells Jane "Tyger,Tyger" before dying.  He also gave Jane a business card with a tiger hand drawn on it.  Now, the barn with the smiley face on it took place in Elliston and much of the story featured Ella's Diner.  Ellis Mars would fit the correct age, too...late 40s or early 50s