On the topic of Metroid stages for 3DS, I can only think of a few: Planet SR388, somewhere else on the Bottle Ship or the abandoned B.S.L Space Station from Fusion. That's what I think.
Also, I saw someone on Miiverse say that "Since Rosalina, the newcomers are being shown in a pattern, with a newcomer from a veteran series, (Rosalina, Greninja, Palutena, Robin and Lucina) followed by a newcomer from a new series (Little Mac, Mii Fighters, Pac-Man). I'm not saying if this might hurt Ridley's chances, but if he gets revealed this month, that'd be interesting.
Please let me know what you think. (Sorry if I can't source the Miiverse poster or account. You know what the Miiverse Smash Community is like... That's sort of why I set an account here... It's a little bit more sensible here... Just a little...)