I am worried it will be someone we have not been exposed to, or we were exposed to only briefly.  In other words: a no one.

OR Red John won't be a person, per se.  But a group of people who a;; claim to be Red John, maybe so if one is captured the work can continue, so Red John's followers can all say they met Red John, but not the same one (and they wouldn't know that).

Personally, I would love it to be Jane's father or Van Pelt's father.