Light type has obviously been brought up too many times, so I'll leave my opinions about how I'd like it to be in the games at that, but another type that'd be cool is Wind type. We need something to differentiate birds/dragons/etc. from other Flying Pokemon, and it'd have type matchups that set it apart from Flying, like:
Wind-type attacks hit Fire, Water, and Steel for half damage.
Attacks hit Electric, Ground, Fighting, Grass for double damage.
Ground hits Wind for no damage.
Rock, Steel, Fighting, Grass, and Electric hit Wind for half damage.
Fire, Water, Ice, and Poison hit for double damage.