That feeling when you found a new favorite Ultra Beast and she finds out.
If I were to make new types, I'd probably do Sound and Cosmic.
A Magic type might be neat too, if it could separate itself enough from Psychic.
...or a Sponge type.
For me, creating a new type would have to balance the current types present, and the only presence is Fairies, in which yes, they are a bit overpowered. An easy fix of that is to make Bug-Types both resist and deal super effective damage against them, in turn giving Bug-Types a proper buff which they could use.
But if I were to make a type myself, it'd be the Anthro-Type, or Human-Type, which I made three years back for my Pokémon game idea that basically was Fairy-Type today. The type was story-relevant and grasped the true idea of evolution.