We know that the biggest draw is the character roster, and so far it’s been growing pretty steadily. Many of those are returning from Super Smash Bros. Brawl, and there’s not a face among them that we didn't expect to see returning to the battle. Yoshi, Mario, Luigi, Peach, Bowser, Link, Donkey Kong, Fox, Pikachu, Pit, Samus, Kirby, Captain Olimar, Toon Link, Sonic the Hedgehog, Marth, Zelda, King Dedede, and several others have been confirmed to return. Not everyone will be exactly the same, though.
Shiek, Zero Suit Samus, and Charizard have all be spun from transformations to full-fledged characters. This change has rippling effects, leading to new abilities for Samus, Zelda, and the characters themselves. Hell, them being stand-alone characters almost means they're new additions, since they'll be rebalanced without their transformations in mind. That’s a hefty list of returning veterans, and it’s still missing some shoo-ins like Captain Falcon and Wario. We suspect we haven’t seen the last of the returning Smashers.