I love the Idea of a Battle Subway stage from Pokemon Black & White, with Trainers as obsticles,
Subway Masters, Generic Trainers, and rivals Bel, Cheren, and N (the later of whom uses the Legendary Reshiram & Zekrom) Dragon Gym Leaders Iris and Shaga, and Game Freak Morimoto (real person, who has an in-game counterpart that can be battled)
The Galaxy Space station Idea is great too, but I'd love to see them incorperate a 360 degree planet or Moon on the Galaxy stage with a black hole where characters can be sucked in and KO'd
I'd also like to see 5 of the classic 9 stages from the 64 version in the game, mostly Princess Peach's Castle, Saffron City, Hyrule Castle, Kongo Jungle, Planet Zebes, and a Sector Z
with updates such as a Team Rocket Helecopter & 3D Pokemon (Johto & Kanto) in Saffron, & upgraded obsticles in levels like Sector Z, Kongo Jungle, and Planet Zebes