I think any of the seven could be Red John. The only ones that match the physical description are Kirkland and Partridge, rest all are either fat, old or a bit tall. Bruno Heller told we will be disappointed when we find out who Red John is and he could already have been cast. Hence it is axiomatic that RJ could be one who is very self-obvious or someone who has not been shown. Hence it could be the two new characters Mccallister, Reede Smith or Partridge or Stiles and not anyone else.