Ghost/Electric! We only have Rotom, and even then it loses its typing when it changes forms. I myself gave that typing to an alternate Mimikyu form (because, of course, it imitates an Electric-type Mon.)
Normal/Ghost could also use further and better representation than that Hisuian edgelord and its babies. Maybe a Pokémon that is explicitly possessed? A regional evolution of an existing Normal-type Mon?
Finally, Ghost/Rock has no Pokémon at all even though there were multiple chances to do one - Sableye, Runerigus, one of the S/V Mons to name a few. I would personally retcon Sableye to have this type because I don't think the Dark-type suits it (plus, it makes Sableye weak to its alleged prey Carbink, which is part Fairy.)