As cool as it would be to do a flying, like you all pointed out it would be too easy. But rock would be just as bad. Especially since they are weak to two of the starter types, which people often use till the end of the game. You add fighting and ground in there, and it just makes a mess. Now, if I had to choose one besides flying, I would go with Alex's suggestion of an electric one or I would go with a fighting. Fighting is only weak to flying and psychic, so It could work. There is also a lot of diversity at both of those types.