The Leviathan Seed (spoilers for Metroid Prime 3)

[Darkurai]

The inside of the Leviathan Seed, if done right, could be an awesome stage.
I guess they could be creative with platforms, since there aren't really any in the Leviathan. There would be no bottom boundries. The boundries would be the top and on the sides, but the side boundries would be somewhat high up due to the hole-openings in the seed. Occasionally, phazon from the core would drop down onto the battlefield. It could be absorbed by fighters to boost their strength and defense, but at a price. The more phazon you absorb, the more you lose control of yourself. Absorb too much phazon, and you will completely lose control over your fighter as they become corrupted, and they'll stay this way until they are KO'd.

 

[Wiseguy]

Cool idea. I like it a lot. However, why not base the stage on Planet Phaze itself?

 

[Jack of Blades!]

Hmm, interesting. However that goes far more into the item usage than stage. It sure would be a unique stage for it to be the only one with that item.