Sylveon, huh?
Well, Sylva/Sylvan in the English language means "wooded", "of part of the woods" (we just a poem with that word in it and had to look up the definition...). However, we already got a grass type.
Sylph could be another combination of the name, which would hint that it would be a flying type since Slyph means "sky beings". Plus, that does fit in with it's french name, which has "nymph" in it.
So obviously Sylveon will have something to do with being a being of the woods. How that will relate to it's type is unknown, but I'm going to say that it is a flying type pokemon. There are so many hints pointing to that, as it is not only the lightest of it's kind, but it seems to have it's name dealing with sylphs and nymphs.
Now, they may have the name refering to as the "protector of the woods", which the nymphs were considered. That would hint that it would be a fighting type.
The german name has feel in it, which makes me think it will have some ability of feelings. Now, I don't know how that would fit in with any type other than psychic, but normal type or the other two types I mentioned makes sense as well.