I'll always hold out hope for a region taking some inspiration from my home region of Cumbria, England. So many mountains and lakes - you know if somewhere's so picturesque a good part of the middle section of Star Wars Episode VII is filmed there, that it's god damn picturesque - would make for an interesting map, particularly if you could then get inspiration from other lake-filled areas around the world for example;
The Great Lakes, perhaps with a Chicago-substitute an a returning Battle Subway?
Lake Victoria inspiring a lake so large you have to get a boat across it - so then there's a big boat like most regions have,
A part based on the Aral Sea - complete with weird pokémon due to the ecological damage, and a mad scientist lab (perhaps because they want to shoehorn Mewtwo into another game, or bring back Genesect, or make another weird pokémon).
I know they technically did the whole lakes thing with Sinnoh but c'mon, they were just identical expanses of water tiles (Although I did like the whole forcibly evaporating a lake thing), with the advancing technologies the possibilities are endless.
EDIT: as for my favourites - either Hoenn because of its difference to what came before, or Kanto-Johto as a continuous whole because, unsurprisingly, it feels so complete and has varying landscapes without feeling like they shoehorned things in without respect to how climate actually works (I'm looking at you, Unova desert).