I have no doubt at this point that an Animal Crossing character will definitely be in Brawl. The Pitfall and Smashville stages have a the leaf as the symbol, instead of the Smash emblem. And the Assist Trophy Dr. Wright has a Smash emblem, but Knuckle Joe has Kirby's symbol. That means while Sim City has no characters that will be playable (hence the Smash emblem for Dr. Wright), but Knuckle Joe has a different symbol because he's from a game in which a related character is playable (Kirby). None of the characters with Smash Emblems in Melee were ever playable through legal means. Therefore an Animal Crossing character will be playable since the compatible item and stage have an unique symbol other than the Smash emblem.
One thing I'm really enthralled about is this new adventure mode. Seeing the movie of Pit in today's update, imagine what it'll be like for all (if not most) of the other characters!