I recently had the thought that Sylveon was a Ghost-type, and after thinking it over, I think it could actually be a possibility.

It's eyes are large and glassy, which have a "nobody's home" look, and glassy eyes are a characteristic sometimes given to dead things in art/literature. Its main color is a pale-ish white, which is very similar to colors used for ghosts. Plus, the whole design gives off a "spirited away" kind of feeling, although that's just me personally.

As for the relation to its names, although it's a bit of a stretch, fairies and sylphs could be considered "spirits" of sorts, which could fall under the Ghost-type.

tl;dr I'm a massive faggot who thinks Sylveon could be a Ghost-type