I know this has been broken down a few dozen times before, but here's my take.
Sylveon is either Rock, Ground, Bug, Steel, Dragon, Normal, Flying, Fighting, Ghost, or Poison obviously. That takes us to 10 types. The whole point of Eevee is that it evolves from the terrible normal typing to a better type, so no normal. I agree anything earthy/dragon seem unlikely. Nothing that looks like that is going to be draconic in any way. Heck, Vaporeon looks more like a dragon than that thing. So that leaves us with Bug, Flying, Fighting, Ghost, and Poison. Fighting also seems unlikely, though let's not forget Meinshao, which looks awfully similar to this thing. But still unlikely I think. Now we have 4. Bug again seems unlikely, nothing screams Bug to me about it and the color scheme seems strange for a bug. 3 left. Poison seems like it would be more...menacing...I suppose. Not a ribbon-covered kitten. Which leaves us with Flying and Ghost. This is part of the wikipedia (sorry for those who don't trust it) definition of a sylph (as in "sylph"eon- "Sylphs are female and are almost transparent. They're slender and graceful, with large glistening eyes..." Pretty accurate depiction don't you think? Ghost is certainly a plausible option, but my vote goes to flying. Now feel free to hate.