Wow... That could possible be true. BUT! Didn't the previous captain/Kenpachi already have a bankai?
But let's say they can indeed pass it on, how? Don't you just "hear it or you don't"?
I like the theorie, but i think it's more that because he surpressed his real power he couldn't hear it. It COULD explain why he just got it now and why Unohana's bankai isn't explained.