Alright.
Since many people tried to guess what will Sylveon's type be, I think it's about time I have a go as well.
Let's go through the most popular options and analyze their strong and weak point, shall we?
BUG
It's said that Sylveon shares a lot of traits with bugs, like its big, vivid eyes and the butterfly-like ribbons. And that's it basically. Even though it would be brilliant to have a bug Eeveelution, that's not the case at all. If that was real, the design would lack some solid insect resemblances that almost every other Bug Pokémon has, like stings, big teeth or large wings; therefore it couldn't perform any Bug move in particular. U-turn maybe? Struggle Bug? But that sounds just..too little.
(on a side note, if it was actually Bug, it would be just perfect for Artie)
Likelihood: 2/10
FLYING
Another interesting theory maintains that Sylveon is a flying type. I can actually see some features that may remind of the wind theme: for instance, the kite-like ribbons and the fact that it weighs less than the other Eeveelutions. But again, this sounds not as it isn't enough to me. It would lack an undeniable flying feature, something which would make it actually able to fly. It has no wings, no clouds, nothing. I can't imagine it floating in the air like Shaymin does which, even if has the same problem in terms of lacking flying devices, can afford it because it's a legendary Pokémon.
Likelihood: 5/10
NORMAL
Normal sounds actually pretty likely to be true, and there are many proofs in behalf of this theory. Two out of four moves Sylveon uses on the teaser are recognized to be Normal typed (Swift&Trump Card), and the design itself looks jolly normal, it is not exaggerated in any ways. In addition, it would close the "Eeveechain" by taking the evolution back to the start.
The fact is...I wouldn't believe a Normal Eeveelution at this point of Eevee's story. It would have been perfect in the first generation, along with the "mutated" ones, but now it's too late to make it able to evolve naturally. Moreover, a normal evolution wouldn't fit its genetic ability to mutate due to the environment in which it lives, because there are no "normal" environments.
Likelihood: 7/10
What do we conclude from that? Well, in my opinion, the time would be ripe to introduce a brand new type.
Why would Gamefreak want to be so reticent about it, if there wasn't something important to hide? And remember, since the Generation VI is basically a "rebooted" Generation II, it would make perfectly sense.
I'm not hoping neither for a Light Type, nor a Magic Type, which could be implemented respectively in Electric and Psychic. What I really expect is a type connected with the inner nature, something like..Emotion. The following list could explain such a choice in a clearer manner:
1) Emotion would not be a redundant type, like most of the made up ones are. Emotion and Psychic may seem interchangeable, but they actually aren't. Psychic is more rational, connected with intelligence and supernatural powers (Metagross and Alakazam, for example), whereas Emotion would be about irrationality, spontaneity and feelings (I can think of Gardevoir, for instance).
2) Some Pokémon would benefit from that new type, because they would be much more "defined": you could immediately think of Victiny's passionate spirit if it was Fire/Emotion rather that Fire/Psychic. Analogously, Pokémon like Chansey, Audino, Luvdisc, Braviary would seem much more appropriate.
3) About Sylveon, it just looks like a possible Emotion Pokémon to me, with all those ribbons and that pink colour pattern. I can see it connected with love (and it wouldn't even go against Eevee's nature, since the trainer himself can be the source of the expositions it needs to evolve), also because it has a heart-like pattern on his face and, well, it was revealed on Valentine's Day. Moreover, the German name "Feelinara" sounds suspiciously like "feeling", which can add an additional part to the puzzle.
What do you think? (: