sharpedo and lawn mower(grass form) rotom
if those were starters however, the water/dark type would have severe disadvantages over it's rival, and there isn't a secondary that can go well with water to nullify the weakness except for ice, with the water dark combination not only would it be weak to both the rival starters types, but also the bug moves that the starter can learn. Starters have to be balanced out and a grass electric would be too broken.If there were a water dark, the grass type would have to be single typed or have the secondary typing to be psychic or ghost. And thus the fire type would be single typed or have fighting paired with it again. I would like to see a game though where the starter pokemon types would be ghost, psychic, or dark, that would be an interesting turn for the games.