I have a theory that's a little out there, but also makes perfect sense.

The way Zangetsu got broken in two, makes me think there is a chance that instead of a katana, he will end up with 2 blades, epic looking daggers or shortswords in fact. Though its a bit hard to imagine his fighting style with two blades, think about it. We've seen two OP captains with two blades, Shunsui and Ukitake. We know they are special, and it's been heavily implied that with 2 zanpakutos that means 2 zanpakuto spirits. So what if Ichigo becomes of wielder of two blades and one is Zangetsu and one is Shirosaki? We know how abilities can evolve and change, actually wielding his hollow power in blade form along with Zangetsu would mean a new range of potential abilities.

We saw Tensa Zangetsu and Shirosaki become one, but now the blade is broken in half. The katana is a manifestion of the wielder's spiritual pressure, and it being broken in bankai means it's forever changed. That change very well could be that Shirosaki and TZ are no longer one.