I have heard several plausible explanations for what Sylveon could be.
1) Normal - It has the whole "pink and fluffy" vibe and stylistically it fits in with many of the more recent "pampered pet" or "best of show" style Pokémon. All previous Eeveelutions have pretty much "screamed" their element, but this one doesn't, which makes sense for a Normal-Type. The attacks it was shown to use included a Fire-Type attack, but Sylpheon doesn't give any other indication of being a Fire-Type, so its somewhat diverse set of attacks may also support this.
2) Flying - The ribbons may be to convey the impression of win, so having it float around on them isn't too odd.
3) Ghost - Its bows appear to be made of its flesh/fir and unless it is just a trick of the art style, they don't wrap around anything; there is simply the bow and the ribbon sticking to the same colored flesh. This could be an attempt at a "cute" ghost; instead of being decomposing and having chains, it gets ribbons and a glowing energy aura (as seen in some of the art).
3) Fighting - less plausible than the first three, think of it as a "violent ribbon dancer". Hey, some of us had to do that in P.E. when we were in school; ribbons totally count as a weapon. @_@
4) Ground - It doesn't not look like a Ground-Type, but that is about it. The thing is, unlike all other Eeveelutions, a Ground-Type seems to be the one where players struggle conceptually. A Bug-Type would be hard to make look right, but being an arthropod-styled Eeveelution would at least fit. Making Eevee more "Ground" based than the other Eeveelutions without it looking like a clone of the likes of Sandshrew, Sandslash, or Diglett is a bit trickier.
5) Dragon - A stretch, but the ribbons and the aura could be an attempt at creating an Dragon-Type that evokes certain draconic traits in a hyper cute fashion.
6) Hybrid - Perhaps this is the first dual-Typed Eeveelution... which even if they only focus on combinations and not permutations (e.g. Fire/Psychic counts the same as Psychic/Fire), there would be 135 potential Eeveelutions.
7) New Family - Eeveelutions might be getting a "Contest/Pageant/Misc." split to further de-emphasize violence in the games; this means this could be an already used Type (like Fire or Psychic), but is part of the "Contest" branch.
8) New Type - While unlikely, it isn't unprecedented. Umbreon debuted in the same generation Darkness-Types did. It has been over 10 years since a new Type was added to the video games, but the taxonomical system is pretty arbitrary, and Sylpheon definitely fits the "magical girl" vibe that is basically its own genre. It makes sense to represent this in the TCG, and similar to how some attacks (and Pokémon) were shifted from one Type to another once a new Type was recognized, former "magical" Pokémon might become dual-Types. A "Magic" or "Light" Type wouldn't be out of the question.