Oh, I have a new idea for a Metroid Stage.
Samus' Spaceship. You battle on top of her Classic ship, as it flies through the galaxy. The rounded shape of the ship would make it pretty unique.
Every now and then, Space Pirates will fly their ships near Samus' ship, creating extra platforms, as well as shooting pirates.
And at times, the ship will land on random planets. She may land in the middle of a lava lake (or, hover above it), so lava sometimes shoots up pillars on either side. And at times it will land right Before Kraid, who will attack the ship.
So many things they could do with this stage, I love it.