Peach's Castle (64), Kongo Jungle, Fountain of Dreams, Rainbow Cruise, Yoshi's Island (Melee), Pirate Ship, Golden Plains, Paper Mario, Gamer, Jungle Hijinks, Orbital Gate Assault, and Woolly World are our remaining returning stages.
Remaining newcomers stages are Gangplank Galleon, Ultra Space, Castle Dracula, Bowser's Castle, and Excite Bike tracks.
All past modes except for Subspace Emissary, Smash Tour, and Street Smash are going to return. There's also a second Adventure Mode that will be featured.
Tourney will come in both Online and Offline modes. Up to 128 contestants may enter.
Captain Toad, Skull Kid, Bandana Dee, K.K. Slider, Isaac, Chibi-Robo, Karate Joe, Zero, Chun-Li, Bub, and Steve are all Assist Trophies.
DLC--should they ever come to pass--will likely not settle for any niche Nintendo characters, but will instead be characters that can, on their own, generate a ton of hype (similar to what happened with Smash 4).
The roster order is 100% accurate, btw. I don't know when, but Simon Belmont will be our next newcomer reveal.