The weakness is easy, Light and Darkness can just be strong at each other just like in most games. Grass can also be one of Light's weakness just like the case of Water as plants absorbs both water and light. This may just empower Grass a little more since its quite weak right now.
Psychic, Electric and maybe even Fire and Grass pokemons may well be able to Light moves and Light pokemons may then well be able to learn Fire and Electric moves too.
If they had the time to seperate Ice from Water, split Earth into Ground and Rock and splitting Ghost and Darkness, I don't see why they can't even come out with Light.
Last edited by Bolter; 11-26-2009 at 11:11 PM.