Perhaps super effective against Dark, weak to fighting. Another cycle like Water/Fire/Grass and Flying/Fighting/Rock. I'm starting to see the possibilities of this new type if it is real.
In this scenario, Fighting beats Fairy, which beats Dark, which beats precisely which one now?
Much as people malign Fighting/Psychic/Dark for dumb reasons, it fits far better.
Anyway, the whole "super-effective-against-each-other-Eeveelutions" thing is probably a coincidence. 9_9 --- Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere
It just seems weird for them to introduce two new Eeveelutions separately when they've always been together.
Granted, seeing as Sylveon could potentially be a new type, it makes a lot of sense for them to introduce a new type with an Eeveelution and maybe they wanted to concentrate on that hype seeing as a new type now is a big deal.
If they introduced two at the same time it would take some attention away from the new type. However, Umbreon was a new type and they still introduced Espeon at the same time. Then again a new type now seems like a bigger deal, plus there was another new type shown at the same time.
Though it wouldn't surprise me at all if Sylveon is the only new Eeveelution. I'll always welcome more though =D!