User blog comment:Tekkphreke/Shikai and Bankai/@comment-24107489-20131225060843/@comment-26420452-20131225071426

There's no other release of the Zanpakutō beyond the Bankai. You know, because Bankai means Final Release.

What Aizen meant that was the true form of Ichigo's Bankai. Just like Byakuya also says Senkei is the true form of Senbonzakura Kageyoshi. That doesn't make it a further release of his Zanpakutō.