Y'know, I've been thinking and I think I've figured out some of the plot-points for this "Unknown Tales" arc.
- Kenpachi wasn't present at the Sokyoku Hill when the spirits left their masters. I'm guessing he'll be important even though he doesn't know his zanpakuto's name. (Just for the record, Zaraki + Bankai = World Over)
- Judging from the episode titles, Zangetsu rebels as well. But I don't believe the "other" Ichigo is a zanpakuto spirit, and he claims that he and Zangetsu fight to become the dominant personality inside Ichigo's mind, so with the Old Man out of Ichigo's head, that leaves "Ichigo" all alone in there. Make of that what you will.
Keep in mind these are only my thoughts and will probably be mercilessly deconstructed by Zan/Jericho/Somebody at some point, but whatever.