"my sword doesent have a name" "my sword is butt naked" -zaraki doesent use anything other than the output of his riatsu. he doesent recognize his sword as an entity. If he doesent do that then he cannot "release a bankai" as the others do. Renji and Ichigo can release bankai because they regonize their sword (they specifically say this in an episode. Unless Zaraki's character changes (which it may not from the direction things are going, it's not his style anyway.) then he will not have a bankai. in bleach he is the only captain who is incapable of using a bankai.
-if there's something in the manga i dont know please tell me.