Now, before I even begin, Sakurai considers Smash Wii U the fifth Smash. As such, I do to.
These are the characters that will be returning. The newcomers will be at the bottom.
Mario: The Mario series will continue to have all of its current reps. However, it is unknown wether Dr. Mario will be a palette swap or not.
Wario/Yoshi: Both Wario and Yoshi will return.
DK: Donkey Kong and Diddy Kong will return, with Diddy's move set being altered to make him a bit less annoying.
Zelda: As with Mario, Zelda will have all of its reps, with Ganondorf finally be fully de-cloned, and Zelda/Sheik will return. Young Link will return with a move set based on the masks of Majora's Mask.
Metroid: Samus and Zero Suit Samus will return, but will remain separate.
Kid Icarus: Pit, Palutena, and Dark Pit will all return, but Dark Pit will be de-cloned.
Punch-Out!!: Little Mac will return.
Game & Watch: Mr. Game & Watch will return.
Fire Emblem: Marth, Ike, Robin, Roy, and Corrin will all return. Lucina will return, just as a palette swap for Marth. However, she gets her own victory animations, a series first (maybe)!
Duck Hunt: Duck Hunt will return.
Kirby: All of the Kirby characters will return.
Star Fox: Fox and Falco will return, and even Wolf returns.
Pokemon: All of Pokemon's reps will return, and Charizard will be back with the Pokemon Trainer. It is unclear if Pichu will return.
R.O.B.: R.O.B. will return.
EarthBound/Mother: Ness and Lucas will return.
F-Zero: Captain Falcon will obviously return.
Animal Crossing: Villager will return.
Pikmin: Olimar will return, with Alph being a semi-clone with Rock Pikmin.
Wii Fit: Wii Fit Trainer will return.
Xenoblade Chronicles: Shulk will return.
PAC-MAN: PAC-MAN will return.
Mega Man: Mega Man will return.
Sonic the Hedgehog: Sonic will return.
Street Fighter: Ryu will return, with Ken being a palette swap, with different voices, taunts, and victory animations! It's almost like Ken's a different character!
FINAL FANTASY VII: Cloud will return.
Bayonetta: Bayonetta will return.
Metal Gear: Snake will not be making any sort of appearance in Smash 6.
Ice Climber: The Ice Climbers will return, however, Wobbling will be removed.
Newcomers!
Mario: Waluigi might be playable. Or he might just be an Assist Trophy again. Probably the latter. However, Paper Mario will join.
Zelda: Tetra will be a playable character, as will Girahim.
DK: King K. Rool will join the battle.
Wario: N/A
Yoshi: N/A
Game & Watch: N/A
Punch-Out!!: N/A
Metroid: A Metroid villan will be playable, I am just unsure which one. It's not Ridley.
Kid Icarus: A character from a new Kid Icarus game will be added.
Fire Emblem: One single character from the latest Fire Emblem will be added.
Duck Hunt: N/A
Kirby: Bandana Dee will be a playable character.
Star Fox: N/A
Pokemon: A new starter from Gen 7 or 8 will be added, and a legendary from any generation will be added, most likely Deoxys or Genesect.
R.O.B.: N/A
Animal Crossing: Isabelle or K.K. Slider might be playable.
EarthBound/Mother: Ninten will be playable.
F-Zero: Black Shadow might join, or Samurai Goroh might be a character.
Pikmin: Alph will be a semi-clone of Olimar.
Wii Fit: N/A
Xenoblade Chronicles: Either Fiora or a character from X will be added.
PAC-MAN: N/A
Mega Man: N/A
Sonic the Hedgehog: N/A
Street Fighter: N/A
FINAL FANATSY VII: N/A
Bayonetta: N/A
New series include:
Golden Sun: Isaac
Splatoon: Inkling
Tales: Lloyd
Shantae: Shantae
Excitebike: Excitebike Racer
Shovel Knight: Shovel Knight