Now I know this is a bump, but the rules also say that if your bump has real relevance, it's OK. And I wanted to share my custom power-up ideas here, which is relevant from this thread's perspective:

Mer-Tail

This power-up is sort of a crossover between the Super Leaf and the Frog Suit. Don this silver scaly fish tail with holes for the arms and eyes which strangely disappear upon putting it on, and not only will you swim much faster and easier, but you can also perform a tail spin attack with a larger radius than Raccoon Mario! Beware though: you're very slow on land, and you can barely jump, but the spin attack still works (but it is slower on land). In games with limited air, this power-up gives unlimited air. In 2D Marios, this power-up state has an L shaped hitbox of 2×2 tiles abovewater and 3×1 underwater (rotated based on 8-directional rotation).
Appears in: water levels. Especially the ones with hardly any to no abovewater action.

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Lakithunder Cloud

After having defeated Lakithunder, he appears in subsequent worlds as a mere enemy with 1 HP. Kill it with fire (or use a Koopa shell I guess) to obtain its cloud. It controls like the normal Lakitu Cloud, but press the attack button and SHAZAM - a lightning bolt crashes into the ground and kills nearby enemies! If you zap a body of water, all enemies making contact with the surface (like Skeeters) will die, dropping a coin. Oh yeah, if you fly into water with the cloud, Mario will take damage due to an electric shock and the cloud disintegrates. Sometimes you need the cloud to power electronic contraptions to reveal stuff like secret exits. While in Lakithunder's Cloud, you cannot tail spin, yeet fireballs or pick up objects.
Appears in: outdoor levels post-Lakithunder boss.

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Bull Cap

Bullcrap pun unintended.

When Mario dons this brown cap with yellowish bull horns and eyes, he can double-tap the attack button (like how you run in Kirby) to briefly charge forward, giving him invincibility to enemies in front of him (in fact, he'll plow through them!) and allowing him to break all blocks (except pipes) that could also be broken with the Mega Mushroom. Kind of a crossover between the Mega Mushroom and Wario's Shoulder Bash. When I came up with this power-up, I was inspired by the Tauros Charge Pokéride in Sun/Moon/Ultra Sun/Ultra Moon. Appears in: mountain levels (and some underground levels).