I think it'd be neat if every Form had an alternate form(like the last form has, spin to switch).
For Energy Form, I'm thinking rapid-fire instead of a charge shot. Or, charging creates a single, slow-moving projectile that does lots of damage.
Nature Form, instead of that thorn tree thing, some kind of vine that grabs enemies (like the tentacles in the Body).
Beast Form, a slower form with higher defense, can't swim through strong currents but can crawl along walls. Attack is the same.
Spirit Form isn't really a form, so I don't think it needs anything else. Also, there's not much to do with Fish Form.
I have a few ideas for Sun Form. It could be an offensive form (since she's light-based, I'm thinking a laser that can be charged), temporary invisibility that you have to charge up, or short-range teleport (light-speed movement, but not through walls, obviously). In any case, the alternate form wouldn't have the light effect like the regular form.
I think that maybe these could be gained by either having Li with you when you gain a Form, or taking Li back to the spot where you gained it initially. Alternate forms would look more Li-like.
Also: A mod that takes place in Aquaria's past, with all the races still living. Maybe just consisting of a few short stories, and in each one you play as a different race.
Also also: More regular songs, to make Naija's regular form more useful. At least some kind of offense. A "charm" song to make an enemy fight for you.
You know, I have a lot of ideas, but I'll probably never actually get around to carrying them out, so anyone who wants to go ahead and do any of this is welcome to it. I haven't even begun to learn modding yet (waiting till I beat the game), but I'm an epic procrastinator.