What I would love to see in Galaxy 2 as a reward for finishing the game is a Playground/Sandbox galaxy that's loaded with different planetoids and obstacles, so you could mess around with the game's gravity system all you wanted.
Some level concepts I'd like to see:
* Planetoids with gravity so strong Mario can't jump (not black holes).
* Planetoids with gravity so weak that careless jumps could send him into space.
* Mario's Luma is removed somehow and so the Spin attack is disabled.
* Planets that move.
* Fleshed-out "indoor" levels, like the mansion in the Ghostly Galaxy.
* A galaxy that's being eaten by a super massive black hole.
* Bowser levels with more varied obstacles and traps.
* Planets surrounded with a gas that confuses Mario, causing him to walk the opposite direction the player inputs.
* Maze levels.
* A planet modeled from the player's icon, whether it be one of the Mario ones or a Mii's head.