I've compiled everything I could dig up in the Direct, plus the 250+ press pictures. Bare in mind I'm only doing series with unconfirmed veterans or new series.
https://www.flickr.com/photos/48280274@N02/sets/72157643678092364/
WarioWare:
The inclusion of Ashley seems to confirm Wario is returning, as she has Wario's symbol. So does Waluigi, even though he's more of a Mario character.
EarthBound/Mother:
The Franklin Badge and Mr. Saturn both return as items. This pretty much confirms as least one of the EarthBound reps is back, most likely Ness.
Kirby:
The Halberd returns as a retro stage in the Wii U version. As we all know, this is Meta Knight's ship, which probably means he's returning.
F-Zero:
As I said before, Samurai Goroh returning seems to indicate Captain Falcon is back.
Pilotwings:
One of Nintendo's oldest series is represent by a biplane. Could a stage and the Rocket Belt mean anything for a playable character?
Nintendogs:
Represented by a single stage in the 3DS version and the bulldog assist trophy. Although doubtful, could one of the dogs be joining?
Find Mii:
Represented by a stage in the 3DS version, as well. But it seems a Mii will always be caged during the fight. It's been unconfirmed if this Mii will be random or not.
Balloon Fight:
Represented with a stage in the 3DS version. Villager's "Balloon Trip" move may conflict with a inclusion of the Balloon Fighter.
Namco:
The most mysterious of the bunch. The Special Flag from Rally-X and a Boss Galaga appear as items. Could this signal that a Namco character is joining in, too?