I am surprised that nobody put a Light-type in any of the Pokemon games yet. They so need one. There just seems to be an imbalance if there's darkness without light.
What would also be really cool is if Light-type's matchups were the exact opposite of the Dark-type. So if Dark < Fighting, then Light > Fighting. If Dark > Psychic, then Light < Psychic. And so on. But then, Psychic attacks would have to be not very effective rather than ineffective against the Dark-type, so that if there was a Dark/Light-type Pokemon, all types of attacks would be neutral to it.