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.
