I have a huge leak regarding this game, which I made this new account to specifically share with you. I got this info from a close friend who knows several people working on the game.
There are several unrevealed items and stages. From New Super Mario Bros. 2, there is a new Golden Mushroom item that turns the receiver gold. Love-love stick, Monado, Ice Flower, and Proto Sheild are in as well.
There are a couple of new stages, including Another Dimension, Chaos Theatre, Popples Islands, Lightning (F-Zero), Planet Wisp, NES Pinball, and Murasame Castle.
All veterans return except for Snake.
Sheik, Zero Suit Samus, and Pokemon Trainer return.
Many unrevealed assist trophies and Pokeball summons exist. These include Dixie Kong, Dillon, T-Rex (Fossil Fighters), Wrestler (Rhythm Heaven), Quick Man, Eggplant Wizard, Yveltal, Chesnaught, Floette, Doublade, Genesect, Reshiram, and Zekrom.
The entire list of remaining NEWCOMERS is:
Lucina
Takamaru
Little Mac
King K. Rool
Palutena
Pac-Man
Galacta Knight
Hyrule Godesses (switch-out trio of Din, Nayru, and Farore.)
The only copy-pasted newcomers are Lucina (Marth), Takamaru (faster Ike), and Little Mac (Captain Falcon).
Mii Mode is a new battle mode. It is a special mode where you set your Mii up with other fighter's attacks. However, no other characters can be fought in this mode, and the Mii can not be used outside of this mode.
Now I'm going to leave this info here, but I can't answer any questions. I know you will call me a troll, or say this info is fake. But I guarantee you will see how true it is when more info is officially revealed.