So, my first real idea for a mod would be to have a series of levels designed around using each form to the full extent of its abilities.
For example, the Nature Form level would require you to use the charge shot to launch yourself places, or as platforms/shields.
For the Beast Form level, I was thinking of having puzzles that would require you to have specific shots from a certain enemy. If possible, this would use pearlgates that are activated by certain enemy shots instead of the Energy Form.
Energy form would be heavily combat-oriented, obviously, but would also have puzzles that require precision and timing, for example, shooting things in a specific order.
Aside from the Forms, I think it'd also be neat to have similar levels for Bind and Shield.
In each level, you would only have the specific form/song required.