That would certainly be a unique evolution method.
Im pretty stumped about it's type honestly, it's very vague. I see it either being Normal, Flying or a new type at this point. Ive seen a lot of people saying "oh Normal type will just be a bigger Eevee" but we do not know that for a fact at all, it's only speculated, it has the chance to be a Normal type. There are some tidbits of evidence for Flying type as well, since it's ribbons were shown blowing in the wind, It's name resembles Sylph, an air elemental spirit, which is said to not have visible wings. Also Ive read Sylvidae is a genus of birds, which may relate to Flying type as well. It also the lightest Eeveelution. Jolteon also does not have an opposite( either weak or strong to it) so that's another thing to think about. Also I may be the only one who notices it's ears look like feathers a bit?
Of course there's also the possibility of a new type. I doubt they are making this big a deal about it for no reason, this implies there is something different about Sylveon in particular, including the possiblity it may be a brand new type, which might explain why it's being introduced by itself. There is also nothing about Sylveon that implies it is any existing type, which is why there is so much debate about what type it is.