Teach Kenpachi the art of killing... Surely can't be him just getting more powerful. Either he has amnesia and she'll make him remember or something that we've not seen before will happen, such as her passing down her skills to him using some ability, or her Zanpakuto(which I feel either has a true shikai form or she has one for killing) or some sort of relation to it, like how Isshin has Engetsu and Ichigo has Zangetsu both with both sharing a common ability.
If Unohana does her Bankai that will probably mark her end. This is if one of them REALLY dies, we know Kubo never likes killing off anyone, though killing Unohana makes sense just like Yamamoto, a high ranking character that isn't all that important to the core story of Bleach.
Well, this is what I'd like to happen, obviously. :D
Edit: Kenpachi should be stronger than he was before no? I mean, Juha's fake demolished him inside out, no way was he not close to death.