Given what we know about Isshin and his zanpaktou(forgive me if that's spelled wrong) we can assume two things about his bankai. First his bankai is incredibly strong, being that Isshin was strong enough to pressure Aizen(without even using shikai) and the revelation that he was a gotei 13 captain(some people say the strongest) and Aizen said his bankai was to tasking on his body for him to use with a substantial injury. Second being that his bankai is a fire type due to the command(Burn) and form(flaming sword) of his shikai. I think it would be cool if he had a bankai that was like amaterasu from Naruto or like blue or white flames(brighter = hotter). what do you think?