Actually, as much as I would hate it, making a Light Type sort of makes sense. We've had Light and Dark in the TCG unofficially since the first Team Rocket series. Then they made Dark an official type. Which, by the way, Umbreon was sort of the mascot for the introduction of a new type. So why not have a Light, with another Eeveelution to usher it in?
I honestly cannot see what advantage there would be to having a Light type... but then again I wasn't exactly on board with Dark, either.
I'm actually really thinking this is what it's going to be. I mean, really. Normally the starts of a new gen are super secret for a while, but these ones came out all at once. So why the sudden secrecy over an Eeveelution? Why don't we get to know it's stats right off the bat like we always have? The design, anyhow, is far too odd to be anything other than Light. Umbreon has red eyes, it would make sense to have it's opposite have blue. Umbreon is rather masculine, forboding, and brooding. Ninfia is feminine, cheery, and quite welcoming in appearance. It is everything Umbreon is not.
I'm willing to put money down that this is exactly what it is. Ninfia is going to introduce us to the new Light type... unfortunately.