I'd say these are the possibilities if there's a character reveal:
If it's a veteran...
Zelda - A Link Between Worlds comes out in less then 2 weeks in Japan, they could very well show her to advertise the game
Diddy Kong - Tropical Freeze is among the games being released near Spring (Even if Feb is still Winter). And the focus is on games that are coming out that soon. They could wait until February's Nintendo direct for this instead however
A Pokemon Veteran reveal - They could pull out at least one answer to the Pokemon representation as all we got from Pokemon X&Y was Xerneas as a Pokeball pokemon
A different Brawl veteran that we don't see coming - There's always the chance that we'll get an unexpected veteran reveal. Perhaps even a character most expect to be cut like Solid Snake? Or we get one of the obvious returnees like Yoshi, Ness, Captain Falcon, Wario, etc.?
If it's a newcomer...
Miis - To promote Wii Sports Club and/or Wii Fit U and/or Wii Party U that have come out. There'll be a storm of reactions in the Smash fanbase if this happens, but it'd also be hype for the casual crowd for the game.
A Mario Newcomer - Probably a stretch but with Mario Kart 8 coming out in the spring (And again the Direct focusing on this) it's a possibility. Maybe even a light of hope for Toad fans that still cling on to hope that Toad's playable despite seemingly convincing evidence that says otherwise.
A long wanted Newcomer for the purpose of hyping the game (Little Mac, Ridley, etc.) - This IS the last Nintendo direct of the year. And with it focusing on games for the next year. They may want to go full force with a newcomer that alot of people have been clamoring for so long.
Pac-Man - Ehhhhhhh consider this a huge stretch that even I wasn't sure I'd put this up as a possibility. But it might be an idea of Nintendo and/or Namco to show off Pac-Man now to hype SSB4 as "That game coming out next year where Mario, Sonic, Megaman, AND Pac-Man can fight!"