Have you guys read this interview?  I came across if after the season 5 finale - back when anything was possible.

http://redblog-thementalist.blogspot.com/2012/06/interview-bruno-heller-teresa-lisbon-is.html

It is a translation of a French interview Heller gave in (I think) 2010 and it contradicts his statements this past fall - to the American media - that he only decided who RJ was in Season 5.  

HERE - he says that Simon Baker and a small circle knew the true identity of RJ from the get go.  He also states that he had a hard time getting Baker to sign on for six seasons, which, if I extrapolate, suggests that his original vision for the show was for six seasons.

He seems to also suggest that the final episode of the series would be the RJ reveal. 

So, either he changed his mind (which would probably have royally pissed off Baker), was pressured to change his vision, or...something else is going on.
 
One can dream.