Look, When a show starts, it is signed up for one season only. All the shows are like that.
Now, the writers must already have planned who was red john was in the first season itself, as they couldn't be sure if the show would get more seasons.
So my theory is that, there's a high probability that Red John was present(and possibly shown) in the first season. It could be anyone like Dr. Linus Wagner or Brett Patridge.
Thoughts?