Board Thread:Questions and Answers/@comment-10797017-20130618215950/@comment-11051098-20130627214302

Kubo did make flashbacks to when Unohana met Kenpachi the first time, and when Sasakibe mastered his Bankai and fought Yamamoto with it. So I think you're right, more flashbacks will probably come.

Out of the oldest captains, we still have, as you mentioned, Kyôraku and Ukitake, whom which we don't really know much about.