I've always thought of Normal Type being the Antithesis to Dark Type how they are both completely Neutral to each other, so a "Light" type sounds a bit cheezy.
Although, They could introduce "Aspects" rather then additional Types, which would add something interesting to the Mix.
Aspects would be like an added Physical/Genetic Characteristic to some Pokemon.
Or maybe Add "Talents" to Pokemon. Talents can Change what Moves that Pokemon can learn or a Pokemon's effectiveness in Using certain moves.