Ya... everybody transforming would kind of suck, because it would be overdone. It would be called the Transform Ball, and not the Smash Ball. Fortunately, I doubt this is the case. I figure only a few characters will actually transform, and the ones that do, probably have some differences, for example, we already known that Giga Bowser transforms briefly, while Zamus transforms permanently, or at least for a long while anyway... some transformations may not need to do anything while it attacks everybody for you, while others you attack the way you want.
In any case, it shouldn't be too similar between characters, otherwise the Final Smash would totally lose it's idea.