I have only watched the first two series of the Mentalist and a couple of episodes from later series but at the moment I think Brett Partridge is a fantastic choice to make. Also I do not know much about the future series so understand whether I make a lot of bad assumptions. Notice how in episode 23 of Season 2 he runs up to Jane has a conversation with him and when Jane's done he turns around and whispers Jerk. It's is mentioned in that conversation that it is someone mocking Red John. Now most people would assume that he was talking about Jane but I think he was talking about the mock killers.
At the end of the episode he shoots his impersonators and says "I do not like people impersonating my cheap work." Or someting along those lines.
Also this is an interesting point I came up with.
http://images3.wikia.nocookie.net/__cb20121021090559/thementalist/images/thumb/2/23/Partridge_pilot....

This is a picture taken from the pilot episode of the Mentalist. While they're examining the crime scene, Jane say's "This isn't Red John, because he thinks of himself as a showman. This isn't the right order."
I think in this picture it looks like Brett is actually trying to subtly compare himself with the smiley face.
The face isn't a sign of arogance though it can be looked at that way. It's Red John sarcastically painting his face.
That's what I think at least
I know about what Lorelei says in a future series, that 'it's surprising you weren't friends the first moment you shook hands.'
Brett works with the police. A very negotiative organisation so he would've shook hands with him, he's obviously not that good of a friend with Jane.
This is something which annoys me about what most people think about Red John and Jane. They're not friends. Red John doesn't think of Jane as a friend, he thinks of him as a worthy opponent. If you put your finger on an idiot's chest and poke it in there face when they look, you're not that impressed with yourself. If you do that to a really clever, intelligent person it makes you proud and vain. That's how Red John thinks.
Thanks for listening.
Cheerio