I do not think the writes from start had a clew who Red John was going to be, so they were making the story as they were going when they found that red John was popular. I might be wrong in this theory but I have some experience with show writers. So looking at clews in much earlier episodes seams pointless to me. Nevertheless I do think there is a possibility this character to be someone who knows Patrick well, someone he will never suspect or we will never suspect. So I exclude all the characters that it is too obvious to be since this will be a let down form the writers and show producers. We should look elsewhere.
Having said all that I must say that the last episode gave me a clue that it is almost too good to be true. Red John is Patrick's father. Now I do not know why I got this feeling since I'm not sure what has happen to his father besides the info the episode gives away but in the great Hollywood tradition it is always a father and son universe. Think Darth Vader-Luke skywalker duo, so why not here too. Now why a father will have the sons wife and daughters killed I have no idea and I can't explain, though I think they can find a reason to cover it if they wish to be creative.