Pokémon X and Pokémon Y

Some people think it might be a light-type, the reverse-type of dark.
I wouldnt' mind that at all, honestly.

Make plant-types immune to it(photosynthesis, durr).
Make it and dark deal super effective damage to eachother.
And well, light-based attacks probably wouldn't be too hard to think of.

They could probably pull the same stunt they did with Bite (Normal > Dark) and make Swift become Light. The big problem with adding a new typing this late would be adding enough of them to have variety in proportion to the total number of pokemon without flooding X and Y with them to a point of nuisance.