Can I just say that threads like this make me love the Pokémon community? Seriously, so many good ideas. I'm hyped now for ideas that probably won't happen, haha! Anyways, I'd like to throw in some ideas to already established ideas since pretty much everything I thought of was said already.
First, I'd love a region based of Mexico. Imagine having to fight the game's legendaries on top of an ancient Mayan or Aztec pyramid. Imagine the evil team attempting to sacrifice the player in a ceremonial battle similar to the sacrifices made to the aztec gods?......Ok maybe not that, but still, Mexico would be cool. Ludicolo and Hawlucha everywhere!
Second, I also like the idea of the England/Ireland/Scotland idea. Imagine the veil team being decedents of knights or something like that at war with the legendary who must be a dragon of course...or a unicorn, dunno...I don't know much about English mythology, sue me. Also, Mega Ageislash made to be a ghostly knight holding the sword and shield meant to be King Arthur.....or maybe the evil team is made up of 12 main members like the knights of the round table... the professor is Merlin! OK I'm getting ahead of myself.
Third, Eastern Australia mixed in with some Outback and the later half of the game being in New Zealand and the post-game in Tazmania or something. I dunno. Platypus Pokémon please...or Steve Irwin as the professor.
Fourth, Africa would make an awesome region! I have nothing to add of substance for that one since I know little about Africa, but yeah. Safari Zone everywhere, yeah!