I feel Nintendo will play it safe and just have us explore One single planet. Because the two most Popular games are Super Metroid and Metroid Prime, where you only explore 1 planet.
Fusion and Especially Other M gets critized for being linear and being Space Stations, so I feel both of those are ruled out.
And in Prime 2 you explore 2 planets (Light and Dark Aether) and in both Prime Hunters and Prime 3 you explore a bunch, but fans don't like those games very much. (They don't like Prime 3 because it's too easy, I liked Prime 3, but I see where they come from)