If you need ideas for your hack, you can get some inspiration from commercial Mario games or even hacks hosted here. Whenever I can't think of ideas I just browse mfgg.net or mariowiki.com and look for something interesting to replicate in SMW. You can also get some ideas by looking through the resources (sprites, blocks, music, etc.) on this site.
Well, since the discussion is back in session over here(I guess. :|) I'll post my thoughts.
Both the Super Mario Galaxy games are wonderful, WONDERFUL additions to the Mario game libary. Making an SMW hack of it should prove difficult. However, if you're running out of ideas, just re-play the original two. It should give you a better feel for the game. Just off the top of my head, I could probably name 2 ideas.
Bullet Battletower Galaxy:
It's a mix between two vertical levels. Using pipes and doors, the player can go in and out of the battletower itself, where the second vertical level comes in. The first vertical level is the tower itself. The thing has many bullet bills, Torpedo Teds(ExGFX to make it look above water) and even Banzai Bills(if you can.) and the primary goal is to get to the top of the tower. In there, you could put a pipe to a horizontal level in there, and end the level with a standard goal tape.
Distorted Space Galaxy:
Basically, it's distorted space. You can use ExGFX to make everything look twisty and wavy, and maybe use a ripple effect. With that, you could have things such as gravity defying water and lava, and the level's gimmick could be based around autoscrolling, and more exact jump precision than usual, with a weaker focus on timing.
If you can guess what levels in Mario Galaxy they're inspired by, you get a cookie.
Floating Forest Galaxy?
With random islands with trees, sort of like a forest if it were say on a broken planet.
-------------------- So remember kids! If you ever get caught having tosave a princess from a foreign land after getting dragged down a drain pipe, jumping while spinning in a circle can help you greatly by allowing you to jump on spinning saw blades, ghosts, and allow you to control your jump better!