All the theories conspire against Partridge:
     -RJ voice is really similar to his
     -William Blake, author of "Tiger Tiger", has a painting "Brace of Partridge"
     -He works at the forensis area, which can make him fake his death 
     -He is very misterious, smart, and has an admiration to RJ
     -Lots of other theories tha you can find on the web

Bruno Heller is a genius. He wrote all the series long, do you really think that the end os the series will be Jane at the FBI doing nothing? No, RJ will be back
The bug on the CBI former agents is obviously a RJ thing. He will be back
This new FBI agent, Jason Wylie, is probably a RJ follower. He is too smart, just got on the FBI special team, and gives RJ Janes informations.

In a few episodes RJ will be back, and in the end, it will be revealed to be Brett Partridge