I have a theory which is somewhat left-of-field, but I think it is plausible. The gist of it is that no single person is Red John, but, rather, it/he is a group of people. Some of the justifications for this are as follows:

1.  Red John has access to multiple law enforcement departments. The most simple explanation for this is that different people step in when different departments are concerned.
2.  Red John seems to be brilliant in multiple disciplines, most notably computer science. 
3.  His individual features have never been corroborated by another person. Given, there are similarities between descriptions, but the people in the final list are also very similar.
4.  The Tyger Tyger conspiracy clearly involves multiple people.
5.  All of the final list of seven could so easily fit the bill, despite some being more appropriate than others.
6.  If multiple people commit the Red John murders, and are in one sense Red John, then this would leave open the possibility that there is another player, not on the list, and thus not expected by any viewers. Therefore. shock value would be increased.
7.  While many people expressly and fervently disavow the notion that anyone on the CBS team could be involved, it is suggested that any one of them could play some part (Cho - strong, silent and keeps his distance from the team (doesn't fit the bill in my opinion); Rigsby - comes across as a bumbling idiot, but this could be an act (possibility); Van Pelt - seems very intelligent, good with computers, more distant than Rigsby, but seems to have an intriguing fascination with Jane (a definite possibility)).
8.  Multiple people would more easily explain how Red John could have deduced the list of seven.

There may be more reasons why I think this, but I have just typed this off the top of my head. The main point is that it is not outside of the realm of possibility that this theory is correct, which would be more surprising.