the theory carries alot of weight and answers a lot of questions, especially as to why ichigos bankai is so different from other shinigamis.
i got a theory also thought i'd put mine in cos its different from urs
if we look at other shinigami bankais ichigo's bankai isnt anything like it, it even confuses Byakuya. other shinigami bankais seem to change the zanpaktou (i.e: make its attacking power and/or attacking speed increase) however it dosnt actually change the user (make him faster or stronger).
ichigos bankai seems to enhance his fighting ability; i.e.: making ichigos attacks stronger, faster; the zanpaktou itself just remains a simple katana and ichigo remains the same physically (he dosnt change physically). also unlike all shinigami ichigo dosnt actually have a shikai. yes at the beginning, when ichigo got zangetsu he called the zanpaktou to release the shikai (that was a cool chapter), however it has remained in the same state ever since. this might be due to ichigo not fully knowing everything about his zanpaktou, but his zanpaktou does go from bankai from to shikai form.
the arrancer all seem to have similar bankais; in the released form of the blade increases the users fighting ability in the form of physically modifying the user. this might be different for the espada but so far this seems to be the trend.
so far we havnt seen any vizards release yet but imho i believe that vizards release their zanpaktou similar to the way ichigo does.
so far it seems ichigos zanpaktou release states are inbetween arrancer and shinigami. ichigos fighting ability is directly increased like the arrancer, but he is not physically changed and does not gain any traits. and ichigo still retains his zanpaktou (still has a weapon separate from his body) and the attacking ability is increased like shinigamis but unlike the shinigami he is directly affected by the bankai, i.e: his speed is increased.
tbh u could make them both into one theory about why ichigos released form is different from both arrancer and shinigami released form.
