Compiled below is a list of CONFIRMED and SOFT-CONFIRMED bosses and character stage hazards:
Yellow Devil (Megaman): Shoots red projectiles from his eye then converts his body into little yellow orbs that fly across the screen where he then recomposes himself. His eye is his only vulnerable point and after taking enough damage, he will explode.
Kamek (New Super Mario Bros U): Kamek will come along and change the landscape of the New Super Mario Bros U stage to 1 of at least 3 different options.
Nabbit (New Super Mario Bros Wii U): Nabbit will run into the New Super Mario Bros U stage and can snatch a character in his bag. It is unclear what happens next, but Nabbit can be attacked.
Dark Emperor (Find Mii): It is unknown what role he will play in the Find Mii stage for 3DS.
Shark (Animal Crossing): The shark attack anyone that gets near him. Which is odd since swimming has been removed.
Ridley (Metroid: Other M): Ridley’s shadow was seen in the Nintendo Direct event from E3, 2014.
Arwing Fighters (Star Fox): Assuming the stage plays like previous Star Fox games, the Arwings in the background will likely shoot lasers at the players.
King K. Rool (Donkey Kong Country): Rumored to be a boss in Smash Run on 3DS.
Kraid (Metroid): Rumored to be a boss in Smash Run on 3DS.
Deadly Six (Sonic Lost World): Rumored to be a boss hazard on the Windy Hill Zone stage for the Wii U.
Bowser Jr: Rumored to be a boss hazard on the Mario Galaxy stage for the Wii U.