^^^ Ya Chozo Ruins is awsome (and would be a sick stage), but I imagine it is something from Metroid Zero Mission, Super Metroid or more likely Metroid Prime 3: Corruption. I'm assuming Ridley will play a huge roll in MP3, because the developers of the game say they plan on adding drama, and Ridley has already been shown in a boss battle. In other words, his course will more than likely be from MP3, and Sakurai obviously has inside information from Nintendo itself.
However, I think the space colony course (hopefully it's a total war zone=uber pwnage!) will be Zamus' course, and Samus will have either Zebes, or something from Prime 1 or 2. I'm guessing there will be 4 Metroid stages, the series has skyrocked since Metroid Prime and Metroid Fusion, though the fact it isn't too popular in Japan is a letback (don't know good games obviously). So overall I expect:
-1 Metroid Prime 1 or 2 stage, maybe 2
-1 Metroid Prime 3 stage, but 3 is more likely IMO
-1 Metroid Hunters or Fusion course
and maybe Zebes from SSB64 or SSBM returning. Plus there is some chance of a Metroid II or Super Metroid stage I suppose, but generally we'll get stages from the 4 Primes, Zero Mission (which I guess counts as The original Metroid, but I assume we'll get a stage that can only be found in Zero Mission), and maybe a Metroid Fusion stage. Just a thought.