Well if we re-examine the original image, we can plainly see pairs forming off.
We know that Umbreon beats Espeon *crosses them off* Leafeon beats Vaporeon *crosses them off* Flareon beats Glaceon *crosses them off* Eevee is probably in there to complete the set *crosses it off*
So far all the arrows are in the clockwise direction, so we can now assume Jolteon beats Sylveon and thus Sylveon is a Flying-Type.
No I'm totally kidding, This theory is utterly crazy and I'd be happy with Sylveon whatever Type it turns out to be. I'd prefer Dragon or Flying, but Gamefreak have done an awesome job in providing no concrete evidence to fuel any speculation in a completely conclusive direction. I can guarantee I'll be getting this Eeveelution in-game now just to see what they've done with its typing, stats and movepool :)