We know that the biggest draw is the character roster, and so far it’s been growing pretty steadily: since E3 2013 in June the confirmed total is at 22. 18 of those are returning from Super Smash Brothers Brawl, and there’s not a face among them that we didn't expect to see returning to the battle.
Mario, Luigi, Peach, Bowser, Link, Donkey Kong, Fox, Pikachu, Pit, Samus, Kirby, Captain Olimar, Toon Link, Sonic the Hedgehog, Marth, Zelda, King Dedede, and most recently Lucario have all been confirmed to return, though we don’t yet know all the changes each character will go through in the transition. That’s a hefty list of returning veterans, and it’s still missing some shoo-ins like Yoshi, Captain Falcon, and Wario. We suspect we haven’t seen the last of the returning Smashers.