Hachi
Happy Valentines Day!
I would say flying, especially after reading what Xinnamin said. But it just seems so much like a normal type its frightening.
Every move it used in this video Pokémon X and Pokémon Y: A New Pokémon Has Been Revealed! - YouTube was normal, and I REALLY hope its not so. I was talking to my brother in-law about it a moment ago and he said it could be Eevee's natural evolution, where you don't need anything to help it evolve, which I almost agreed. But the problem with this is Espion and Umbrion both evolve by love, so you would have to try to make her hate you to get her to the right level. Maybe some sort of item to keep her from getting to happy, or they could change the way you evolve Eevee into Espion and Umbrion. I think that would be sorta cool, but I still would rather not have a normal type :p(unless its part flying of course ha ha)
Espeon and Umbreon evolving by affection really wouldn't affect Sylveon as they are plenty of ways to get around that. Think Happiny who evolves by leveling up at a certain affection level while holding a Oval stone. That would override the time of day affecting which eon your eevee would evolve into and automatically guarantee you a Sylveon. I actually do think the affection route has a pretty strong chance considering the new battle mechanic involving having stronger bonds with your Pokemon = stronger Pokemon in general. Sylveon could very well be the poster child for showing off this new mechanic, in the same way Umbreon and Espeon did in Gold and Silver.
I'm honestly keeping my mouth shut about the type. Nintendo is throwing us for a loop for a reason, so I wouldn't be looking for answers in the video- Plus battle animations can be ambiguous. Anyone remember the Dark > Physic > Fighting starters rumors going strictly based on just the color of the attack in two of the instances? My best guess is Normal, but I kind of want it to be bug for some reason... just because more awesome Pokemon need that typing. We got a few really good ones in Gen V, hope the trend continues even if Sylveon ends up not being one of them.
Edit: Sorry if anything above has already been said; I'm at work so I can't really read through the comments too much.