User blog comment:Xilinoc/Ch. 621 - THE DARK CURTAIN/@comment-7606564-20150326123608/@comment-10497166-20150327064019

I'm pretty sure aizen has a bankai. Why? Bcoz if u remember his main reason for doing all that he did was to become more and more powerful. In fact the only reason he transcended was bcoz he had become the strongest he could in his normal shinigami mode. If he hadnt achieved bankai dont u think he would work on that first to become more powerful? Now if the writer still decides to go with ur theory, then another possible explanation for him not doing everything i just said could be this: As u know, u have to fight ur zanpakuto spirit to achieve bankai. Aizen's shikai itself is so effing powerful, then imagine what his bankai would be like and him having to win against such powerful illusions would be very difficult. So maybe he was unable to win against his Zanpakuto and that's why he decided to increase his powers just so he could beat his zanpakuto and achieve bankai.

Second explanation is unlikely but possible and would make for a great plot line if u ask me.What do u think?