Z: ?? (I know it's a bit early)
Perfect counters assuming Fairy is also SE against Dark.
Dark isn't weak to Fairy. In the trailer, Gardevoir (who used a probably special move and is known for Sp. At.) hit a Hydreigon with a Fairy-type attack and it only brought it down to about 2/3 health. If it were 4X effectiveness, Hydreigon would have fainted.
Honestly, I agree with you about Yveltal but I see Xerneas being an Ice/Steel type or a Grass/Steel type mainly because its legs look like swords to me.