Here's a little tidbit from the Japanese website:

http://www.nintendo.co.jp/wii/rmgj/galaxy/index.html

Talks about the levels a little bit, in Japanese though. The third level looks as though it may be a Bowser fight. If you look at the first picture of that level at the top right it looks as though you may be shot there with one of those stars and fight Bowser. Hopefully its like Super Mario 64 where you fought him 3 times, instead of Sunshine where you fought him once and he didn't even move.