While I feel that Brett Partridge was originally conceived as Red John and then moved aside, the character of Ron Lastnameless also makes a nice suspect. He perfectly fits the concept of the half-invisible background mole, present for a long time throughout the series.
Remember Red John's alias, "Roy Tagliaferro"? Roy Cut-Iron? Well, here's a funny little word shuffle:
Cut iron...
Iron...
IRON...
CBI Ron...
CB IRON.
:)
It's quite likely that unless Bruno Heller gets tired and abandons the concept, he will eventually reveal Ron as either Red John or his very important minion.
How do you find this theory?
plausible
unlikely