Light, as in "holiness" and "purity" wouldn't fit well with Pokemon, I agree. But light as in the light spectrum would. Could you imagine a flashlight Rotom? lolz
I could easily see a Light-type being weak to the types that are underrepresented offensively, and resistant to the stronger ones.
But if Sylveon isn't a new type, my hope in there being a new type will diminish somewhat cause I can't imagine a better way to introduce a new type than with an eeveelution.