Given the likely chance of being set in France, I would love it if they had something like the
Catacombs of Paris. A wall of bones and skulls, tombstones with ancient writing, there's so much that can be done with an underground cemetery such as this. And if they can make it as half as eery and haunting as the Catacombs of Paris, then I'd bow down to GameFreak's feet.
...figuratively speaking.
Also, it'd be a great place for the region fossils! Ghost Fossil Pokémon, anyone?