Derr, how about Rock? Or Poison?I really hope I'm wrong, but there is no other type that can make the third legend weak to Dragon and Ground and *insert another type here* so far...though thankfully Light weak to Ground wouldn't make much sense anyways :P And that's also assuming that there WILL be another legendary trio like the past two generations.
Come to think of it a Rock/Dragon or Poison/Dragon would be pretty cool...
Also, watch Meguroko not get Pursuit =/