^^ I dont know Paul. Unohana knows the "Art of Killing" and she's about to teach Kenpachi (should i start to call him Zaraki now since there are two Kenpachi's). I imagine that "the Art of Killing" is not related to a Bankai, so they wont have to worry about anything being stolen.

With that being said, you'll have a powered up Zaraki, that shouldnt get man-handled by Bach, unlike before. Plus, you have another Kenpachi, who is arguably the strongest Shinigami of them all.

If Mayuri or Urahara figure out a way to block the Bankai stealing tech. Then you still have Shinsui and his Bankai, which must be powerful since he was chosen new CC.