You know what I want to see out of Mario Galaxy 2? No? Well, too bad! ;)
1. Drop the 120 stars - After playing Mario 64; Mario Sunshine; and Mario Galaxy, I really want this tired remnant of old design to finally go. Especially after playing Psychonauts recently, I'm done with "collecting" in games and collecting Stars to open worlds, etc. hasn't been fun since Mario 64. It just gets so tedious to select a star, run through a level, do whatever to get the star, go back out to the hub world, and wash; rinse; repeat. Make the game Mario World-style with a series of linear levels you can run through with secret exits leading off to secret galaxies, etc. No collecting crap, just run straight on through.
2. Allow multiple characters right from the outset, just like in Mario 2 - I have no idea why this idea hasn't been used since Mario 2, but it was a really nice one. Let us pick from Mario, Luigi, Peach (damn lazy, useless ***** finally has to work for once!), etc. and off we go into each stage! Some secret exits can only be accessed by particular characters, giving each a unique purpose. Yoshi can stay as he is: a character each of the mains can ride. Perhaps he gains different abilities based on who is riding him?
3. Make the main quest an odyssey to kill of the biggest menance to ever hit the Mario Universe: Bowser Jr. Enough said.
So yeah, basically make it an expanded New Super Mario Bros. Wii in space in 3D. ;)