The new Mii Costumes are:
KK Slider
Toad
Skeleton outfit (Looks like Dry Bones to me)
Mega Man Volnutt
Dante (DMC1)
Chun Li
Beat
Billy Hatcher
NiGHTS
Klona
Akira (Blue/Red outfit)
Dig Dug
K. Rool IS playable! During the Smash Direct, Sakurai mentions that he made the Mii Costume to see if people would be interested in buying it. Not too many people did, so he pushed K. Rool to release in 3 months. gg guys
Wonder Red is also a newcomer. He has palettes based on the other characters in W101, but not full costumes.
Shantae Mii Costume is confirmed for both Mii genders. Sakurai mentioned that she was "very uninteresting" when thinking of a moveset for her, so he demoted her to costume.
The Mighty Morphin Power Rangers (Sakurai refers to them as Zyuranger) appear as an assist trophy.
Sakurai tells the stream that he did try to make Ridley work, but ultimately scrapped his work.
Alternate Costumes are indeed coming! Due to the large cast, they will be coming with a later update/patch. He showed off Mario, Captain Falcon and Link. Mario had the Sunshine glasses and shirt, Captain Falcon is his SNES outfit, and Link is in his casual clothing from Skyward Sword.
Sakurai mentions that Bayonetta was considered, but dropped in favor of Ryu
Sakurai also mentions that custom moves and dialogue for the DLC characters will come once they are all wrapped up.
Goku's outcry has been noticed by the staff, but will not be put in the game. Sakurai hints at the possibility of a mii costume?
This is alot to take in.