I see no reason why it could possibly be Shisui (or Kisame). The main reason why i totally discount it is that Shisui is the son of Kagimi who was one of the seconds closest followers. The second has been shown to be one of the best sensor Ninja's out there - he was easily able to dis-concern that Karin was an Uzumaki simply by feeling her chakra and he sensed Naruto go into sage mode. So to think he would not recognize one of his key followers children is a little beyond belief at the moment.
Kabuto changing his ways makes perfect sense. He was put in a technique that is specifically designed to make you change your way, so i think calling it retarded is going a little far.
But I dont think it is Kabuto either. Oro or Karin should have been able to sense him even if he had changed alot and if it is Kabuto his change would have probably taken away the malice he feels towards Sasuke, but this guys chakra is not welcoming. Oro had known him long before he changed his body, and Karin had spent enough time with him to know his chakra - remember she was not a prisoner like Sigetsu, she would have spent a fair bit of time with him and he cant change his Chakra THAT much that he could avoid Karin's abilities. Kishi will simply pluck out someone who Sasuke met whilst away form the village and give it a flashback or it will be someone from ages ago like those Sound nerds from the Chunin exams.