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Wave Type is a obvious type that needs to be introduced. This would be perfect type to introduce dual typed attacks.
Wave as a element stands for anything involving the waves in our space that moves things. And when you pair it with certain kinds of elements, it becomes Light, Electromagnetic, Radiation, Vibrations and Sound. And in that case, Electromagnetic, Radiation, Vibrations and Sound attacks become a real thing. Even that, it would fit prospective Pokémon that have similar elements.
It would be effective against to Rock, Steel, Ground, Ice, Dark, Psychic and Fighting because of how waves affect materials and minds.
However, it wouldn't be effective to Water, Electric and Ghost with how flexible and adaptable these elements are.
Despite that, Psychic and Electric would be the types that remains effective to Wave as they'd probably have manipulation over the mentioned type that makes it weaker.