This is exactly what I'm thinking. Why do we need a Light type to oppose Dark? There are already things that Dark is weak to.. it's not like it's overpowered or anything.
They didn't bring in new types "just because" they did it because Psychic was way overpowered and they wanted to balance everything out. I don't think that adding a new type would help balance anything, it would probably actually ruin a lot of things because a lot of past Pokemon would need re-typing, and it just seems like a lot of unnecessary effort, I don't think there's going to be a new typing =/