As for a suggestion for a "falling" level, the level could be similar to the Ice Climber's level in melee where it automatically scrolls. You could have a bunch of platforms falling down from the top and you can land on those to attack, and you keep jumping up from platform to platform.
As for my idea on the stages, I would hope the castle one is from Fire Emblem, just because I want Marth to return, and for him to have his own stage. And since it seems that there is an inside and outside, it could mean either the level will move, or there's some way to enter/exit the castle, or it will be two different levels.
The Hangar with Metaknights ship the Halberd looks like it will be like Mute City, and you'll fight on one platform as it takes you around the level. Which looks pretty cool too.
As for the rest that we don't know about, I really hope that they add more "gimmicky" stages. I'm a great melee player (or was back in the day) but I'm not a tournament kind of guy, so I'm all for crazy stages. Things like Hyrule Temple, or Big Blue. They just make the game interesting and more fun. I wouldn't mind a few simple ones, but I hope the majority have things going on around you.