I think its Kabuto. Like the topic starter stated, Kabuto is one of the best Medical Ninja around so he could save Sasuke. On top of that it would be really stupid to bring in a new character, or even a character that we know is already dead. A lot of people are saying it cant be Kabuto because of Karin saying there is a "bad vibe" and that Kabuto should be "good" now. But really, Itachi said that the way to escape Izanami is to be true to the self and accept reality. It says nothing about turning a new leaf and being good. So maybe his true self would still have a cynical vibe to it we dont know.
The other most likely possibility is a white Zetsu just because they're everywhere.
The other most likely possibility is a white Zetsu just because they're everywhere.