I want Galaxy 3, but just do away with the space motif. You can even sense in Galaxy 2 that being in outer space is being used less. It's no longer needed to make the player understand why the games mechanics work the way they do. Just stick with the anything goes method and run from there. Personally, I think Galaxy 2's method of having tons of small levels works brilliantly. This is definitely the closest I've felt to playing a 2D mario in a 3D mario and I, for one, think that's good. Don't get me wrong, Super Mario 64 is one of the greatest games ever, but Galaxy 2 has a very pick-up-and-go feel that the others don't have as much. SMG2 provides more instant satisfaction than the others.
And I think the reason galaxies in SMG2 may not be standing out as much is, honestly, because so many are good. Freezy Flake, Shiverburn, Melty Monster, Sweet Treat, any boss level... it goes on and on. Only a handful of galaxies in the game didn't blow me away.
edit- as for areas to improve... I can really only ask for more of what's already working. Some more throwback levels would be an obvious hit. And, frankly, I don't think expanding beyond the 120 star formula would be bad. Why not make it 150? You can already tell with the recent trend that Nintendo really wants to make their Mario games bigger, so I say they should do it in a more direct way. With all the features in Galaxy... I can't really think of other things that I would want that wouldn't be completely silly...