I hate to admit it will be Jane because I admire him so much and learned so much from him, but the fact is, there is a high possibility he is Red John himself.

In Season 4 & 5, we can see that Lisbon is slowly becoming an apprentice to Jane. She picked up some of the mentalist skills and been improve whether in terms of observation or manipulation. Jane also constantly shows hints as one day he might be gone.

Throughout all the seasons, we can see Jane staring into the mirrors and somehow becomes another person for a split second. This might be because he has split personalities and he himself does not know about it. When he becomes Red John, he might black out or something.

My guess of the ending is that, in the end, Lisbon found out Jane is Red John, and she has to use what she know and learn from Jane previously to take him down while Jane realized who he is but unable to control himself. And in the end Lisbon outsmart Jane/Red John and confronts him and kills him. Jane will then thanks Lisbon for killing Red John in his dying breath.

I know there are some arguments to this theory that Jane is actually RJ such as shaking hands before and stuff. The thing is, no one knows for sure the person they see is really RJ. Maybe Jane ask a person to pretend to be RJ and directs the disciples to do his bidding but actually this 'RJ' is also one of the disciple who is acting as a cover for the real RJ aka Jane?