My guess is that Sasuke will be evil and train, but Naruto will end up being the one to slay Itachi. This will enrage Sauske ever further. He and Naruto will fight. Naruto will pwn him and while getting better, Sauske will be reabilitated both physically and emotionally. Something will happen to Naruto (fighting Akatsuki) and Sauske will come and save him.