Yoruichi's Zanpakutō might have something to do with the fact that she can transform to a cat. Maybe it's power is transformation (at least, it's shikai) and as a result we never see it per se, because it's always in a released state, being some other form that Yourichi takes. The cat might be just one of them. She might be capable of transforming in anyone, a skill very useful for the Onmitsukidō ex-Commander-in-Chief. her power might be similar or greater than that of Sternritters Loyd Lloyd and Royd Lloyd. What do you think?