I know he is not on the list, I`m not an idiot. If RED JOHN REALLY IS ON THE 7 SUSPECTS LIST, this is not a theory, but just an interesting fact that came to my mind.
You know how all RJ`s accomplices always say that is a charming person who can play with people`s minds. Well, Mashburn even got Teresa Lisbon to sleep with him. I mean, we know that she has a thing for Jane, so it is really difficult to make her sleep with you all of a sudden.
He always gets what he want. I think we all think at Red John as an ugly creepy person, but clues and his accomplices`s statements prove that he could be, actually, even more charming than Jane is.
I say it again, I`m not accusing W. Mashburn of being Red John, but, in my opinion, he is the most charming person (except Jane, of course) of the show. He could make any women fall from him, any man feel "small and intimidated" by his power and charisma, and he could also make kids love him. He is smart, rich and funny. Nothing like Red John. This is why he could be, from this perspective, the most obvious suspect.
Of course, he also has a thing for red sweaters. Even his employees wear red shirts. But this may be just a coincidence. Anyway, I`m sure that Mashburn is not just another character in the show.