I'm liking the idea of a light type and do not believe that it will overall affect the current typing too much:
In reference to the people mentioning that light would only beat dark and vice versa, dragon only beats dragon, but is also weak to ice?
Pokemon that carry the dragon type are usually, if not always, very strong in offense terms, and then their dual typing defines them and provides counters.
What if light could be a more defensive "ultimate" type? We already have a few defensive types (Poison or Steel for example)
It could be strong and weak to dark types, and potentially also strong against ghosts. Similar to dragon but in reverse as dragon has 2 weaknesses; light would have 2 strengths.
This nature of the typing (2 strengths 1 weakness) would probably mean however that light would always be a secondary typing - Sylveon maybe light/normal?
But then the pokemon carrying the light type would be in general more defensive and not big attackers to counter this typing and then the dual typing would provide further strengths and weaknesses.
This could then be applied to the new legendaries perhaps? Xerneas and Yveltal could be light-dragon and dark-dragon meaning that they are the perfect counter for each other.
Xerneas would be weak to dragon, ice and dark. It would then take lesser damage from fire, grass, water, electric and ghost.
And Yveltal would be basically identical to Hydreigon except with the addition of being weak and strong to light.
So therefore I believe it is definitely plausible, and it will really shake things up a bit.
But yes, it will vary a few things in competitive play but there will be new moves to accompany the typing so it'll just be new and different!