ah. So that's what's happening in those "long run video" of WADF when a menu appear on the left ... the ball is changing what it is made of.
And, well, it's indeed not quite unique. Even just in the Super Mario series, we've seen "steel mario" that cracks things and be heavy enough to walk underwater. There's also the 'mario babies' of "Partners in Times" that can be flattened to access secret area and fly because of their new shape. If instead i make Bilou capable of bashing bricks with his head or throw his punch ahead to smash monsters, it's really looking too close from something super-known. And no, i don't feel like giving him sci-fi power-ups (the game is supposed to involve little magic from the "magic gems" Bilou collects after each boss), nor do i think flappy wings and bow are a good suit for Bilou (that's Kirby's job, after all).
Well, this is still under discussion with my brother (co-scenarist) anyway, but thanks for mentionning the likeliness.
Imho the "change material" has the advantage to bring consistency in power-ups (just like Mario in SMB3 gets his super-abilities by changing costume)