I think Red John is Jane's brother.
I dont know anyone mentioned about it but the tiger poem says "what immortal hand or eye could frame thy fearful symmetry"
So i think that symmetry is Jane's brother. Twin maybe
Ok thas was really far guess I accept that
But its an option.
My second man is Brett Partridge.
His voice is the same as Red John.
Not only his voice. Whenever we have a fake Red John case he shows up to investigate.
And Jack Plotnick has a perfect villain face to be Red John.
If Jane is Red John that would be so simple. I dont think scenarists would do that.
Im watching 5x05 right now. So these are my guesses so far.