I just thought of an idea that would make these powerups not overpowered! I was just planning on them being usable until the rest of the battle, but their attacks would obviously be powerful because they would only be playable if their respective powerup was purchased in the shop. I call it "powerup points". Once you consume the item and transform, you'll get powerup points. Each of the transformation's attacks will cost powerup points. If you run out, you turn back into the original character. Sounds pretty balanced, right? Now let's talk about those pictures:

Fire Mario: He'd have pretty standard attacks. His first one would have you press Z repeatedly to charge up a fireball, making it bigger. If it got too big, it would explode.

Tanooki Mario: One of his attacks would be similar to Wario's ground pound, where he'd jump and then turn into stone to crush the enemy.

Propeller Mario: His attacks include spinning and more spinning. Did I mention spinning?

Boo Mario: He's never spit spirit fire before like some boos do, I'd probably add that to his arsenal. One of his attacks would be teleporting behind the enemy and spooking them.

Rock Mario: Of course one of my favorite powerups had to be planned. He'd curl up into a giant rock ball and roll over the enemy.

Snow Mario: This is one of the custom powerups I found. In fact, it's the only one I have that's worth keeping at the moment lol. He'd have a stronger jump and be able to throw snowballs.

It would be easier to just make them one move instead of fully playable (actually maybe I should do that. I'll think about it) but that wouldn't be fun. Cya next time!