I finally got around to listening to a few more of these. Galaxy 2 is still my favorite game on the Wii. It outshines the original Galaxy since the boss battles tend to lean a bit harder, but I sort of enjoy the atmosphere and music from the first game a little more. I still like to hum the themes from Battlerock and Buoy Base quite a bit; while the music from Galaxy 2 is also superb, I have a more difficult time remembering it.
As for future installments, I've wondered a lot about where the series might go from here. Will continue to focus on a smaller setting, such as Super Mario 3D Land or a setting more like Super Mario 64? I know it won't happen, but now would be an excellent time for them to bring Mario back to a setting like the Dream World subcon (like in Super Mario USA or the western version of SMB2). I think that a dream world setting could be sufficiently varied and abstract enough to continue the crazy world design that the Super Mario Galaxy series sports. Plus, re-exploring other villains such as Wart, Birdo, and Mouser would hit the nostalgia button for many of us older Mario fans who grew up playing and loving SMB2. (When it was released, I knew nothing of the game's true origins.) Getting away from the Bowser and Bowser Jr. as the main villains would help the game seem a little more fresh and interesting. I think the western markets would eat it up, but that's just IMHO.