Tbh, I feel like some of the legendary typings just seem kind of arbitrary and nonsensical. Nothing about Nihilego’s design suggests rock type at all - if I had to guess I would probably say it’s psychic/poison. Rock/poison would make an interesting typing for a regional Crustle whose shell is a chunk of uranium ore.
It’s the same with the Gen 5 dragons - like, why exactly are their second typings fire, lightning and ice when nothing about their designs really evoke those elemental properties?
Palkia too. I guess pearls are found in water, sure, but then why not give it a more water-inspired design? Making it an aquatic dragon/sea serpent wouldn’t hurt.
The one unused type I want to see is poison/steel. These two types initially seem at odds with each other but have a lot of potentially interesting concepts. The most obvious is something based on mercury, but you could also have some sort of machine/pollution based pokemon.