Ok, as we know, a large amount of shinigami, arrancar and vizard characters haven't had their bankai, their initail release, or even their zanpakutou shown. This thread is for theories about what the zanpakutou's releases and abilities are.
You are allowed to be as crazy as you want with the theorizing.
Personally, I think Yamamoto's Bankai would be a giant nuclear bomb blast. It would destroy anything in a 10-mile radius. Therefore, he would be killed in it, so he doesn't use it except in serious circumstances.