What else would they be intended for? You don't think Game Freak's decision to make a Pokémon evolve into eight different beings of different types was a conscious decision? Do you think the fact that they were all given a different, single-typed designation was by chance? I don't understand where you're coming from; its quite obvious that Eevee is a vehicle to represent the different types.
But when I look at Ninfia, the flying type doesn't jump out at me at all. Nor does the bug type. And I highly doubt there would be a Normal typed Eeveelution, as that would be a dull and uninspired design decision to say the least. I don't dismiss the possibility, however. But I still say that Light or some other new type is the most probable.
Given that Dark type is actually the "Evil" type in Japan, Light type would be "Purity" or something similar, as that is the logical opposite. Not actual light energy as the two of you may be imagining. That said, Purity would best be represented by gentle colors (not too bright, no clashing), delicate disposition, feminine or child-like qualities, and an overall welcoming, nonviolent stance. Similar to Chansey/Blissey and Togepi/Togetic/Togekiss. Probably a lot of pink, probably a predominance of hearts in many of the designs. Their moves wouldn't focus a lot on attacking, but rather securing defenses, healing, and strengthening teammates. Do you not agree that that is probably what Light types would probably look like? And does such a description not fit the new Ninfia better than any type we have already?