I'll give some initial thoughts.
It looks like we're getting smaller stages that kinda goes with the smaller screen. This could be very good for us, as I'm already seeing the Wii U stages and they're HUGE. This may actually make our game a tad faster as we'll be able to kill easier. All good things. The game itself might even just be a tad faster for us do to all of this, we may really have a better game for certain things.
Stages will depend a lot on how we feel and how connecting works both locally and online. If it's good, we could definitely be a little more liberal on things and have some more stages.
Tortimer's Island could maybe work, it depends on how the edges work. I haven't seen anyone grabbing onto anything in the videos. The dock looks like it could be, so even if the other side isn't we had Frigate legal with a missing ledge, and the stage is very tame either way.
Reset Forest is a mixed bag, it all depends on if we can abuse how odd that second transformation is. I think the first one would work just fine for competitive play, even with the hole in the middle. Just don't pick Ness
Prism Tower and Arena Ferox look a ok for legal. All of the others really have some exploitative possibilities or have crazy strong hazards.
Smash Run while not something I'm personally going to use a lot looks like it'll be seriously interesting. The items and equiping them for other fights will be something we have to consider for competitive purposes, it may be very difficult to implement but since we can pre store things on the 3DS it might not be impossible either. Only time will tell.