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aizen isn't all knowing. he didn't even seem to know about the royal guard initially. he was still dabbling in hollowification when urahra had already perfected it by creating the hogyoku (a feat aizen couldn't duplicate even with another 100 years, instead needing to steal the original).
there very well could be more about shinigami that aizen doesn't know. perhaps a powerup that doesn't come directly from a zanpakuto or perhaps learning additional stages of bankai. byakuya has three stages of bankai at least so i'm sure others could learn multiple stages as well (and the very existence of said stages make the name 'final' seem rather dubious).
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