If the Pokedex does get expanded beyond the Platinum Dex, two areas I'd like to see more variety is the Old Chateau and the Lost tower. These would be good places to have more ghost types if they are added as I think they have always felt very limited in options which was understandable at the time but if the Pokedex was expanded that for me would be a great time to improve these areas. (as I do like the slightly creepier areas like this)