But again... I really do hope it's going to be a brand new type.
I totally agree with the Sylveon being connected to forests based on the the name being based off of the Latin word "silva", meaning forest. Seeing as all the bug-types were mainly located in the forest sections, and the big butterfree-esque eyes, I'm going to say say bug-type. Also, as pokemon generations progressed buy-type stats and advantages have seemed to grow (at least to me). I think that it would be a unique addition. I can see the strength of the normal-type evolution argument, but it's just more fun for me to think it will be the underdog: bug-type!