http://bulbapedia.bulbagarden.net/wiki/Dark_Pok%C3%A9mon_%28TCG%29%20and http://bulbapedia.bulbagarden.net/wiki/Light_Pok%C3%A9mon_%28TCG%29
I don't know what you guys could do with these Light and Darks, But i think you could someting crazy, Like turn some special attackers into physical attackers or the other way around, or make a tanky pokemon like shuckle into a speedy sweeper (2 broken?) Something along those lines, swap up the base stats or something, lets use mewtwo as a base
Turn it from a Speedy SpAtker, into having a higher base atk stat so it could be a better physical attacker, or for Dark form, turn it into a Speedy Tank with lowered offenses but drastic increases into Defences, see what i'm getting at?