ttestagr said:
The only abilities that had a limit that Aizen mentioned were Zanjutsu, Hakuda, Hohou, and Kidou. He did not mention bankai at all. Also, it is incredibly rare for a shinigami to have the potential to achieve bankai. One or two per generation I believe it was said. It makes sense to me that the reason Aizen pushed himself to reach the limits of the shinigami arts and then go further would be because he just doesn't have the ability for bankai.