So Sylveon is pretty much assuredly based on a fairy. The english name comes from
Sylph, a fairy-like wind elemental, the french and japanese names come from
nymph which is some kind of fairy and the german name comes from the word Fee, which means fairy.
With the fairy theme it has going, I would assume that Sylveon could be either one of those types: normal (most "fairy" Pokemon such as Clefairy, Chansey or Snubull are normal type), ghost (a fairy is a sort of siprit of the woods), bug (western fairies are small and often bug-like), flying (They fly, also a sylph is a wind fairy), grass (fairies dwell in the woods), psychic (magical powers), or an altogether new type.
Psychic and grass are already taken so that leaves us with normal, ghost, flying and the new type. Now it really look like a bug and the lack of wings makes me dubious about flying. I really don't think they would make a normal type Evolution since the whole point of Eevee changing type is because it adapts to its environment. Ghost would be the most logical choice since it's supposedly based on nymphs/sylphs/wood spirits. However it doesn't look too ghostly to me, especially with that colour scheme.
That's why I personally think that it will be a new type. What better way to introduce a new type than with a new Eveelution? They've done it before with Umbreon and the dark type. The 2 most popular theories regarding a new type are the light type or the fairy type. I for one think that a fairy or magic type would make the most sense. It could probably act as a counterpart to dragon, as they're both mythological creatures.