If you remember when Kyoraku and Ukitake were fighting against Starkk, the 1st Espada, Kyoraku was on the verge of using his bankai, but Ukitake intervened and said "not to use it where others can see it". Now, Kyoraku is in the darkest depths of Muken, where the only one there is Aizen. If a dispute were to occur between Aizen and Kyoraku, I believe that it would be the perfect opportunity for Kyoraku to use his bankai. What do you people think about this? 