I think after last generation and the introduction of Fairy types to add to the other seventeen, it'll be a while before GF adds another, if they ever do. And Dark already has three weaknesses, fighting, bug and the new fairies. Another type, which many Pokémon can fit quite well, would probably leave it outclassed without buffs to balance it out.
Also, I have a feeling the new Moon legendary is gonna be Dark/Fairy. Dark honestly just fits the best because of the whole moon night sky wings thing. While Flying would fit as a secondary type, they already had Yveltal last gen and I doubt they'd repeat it. Rather the secondary type would be fairy, after all moves like Moonlight and Moonblast are all fairy type and would receive STAB too.