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Reposting from the last thread we had on this.
Region loosely based on Britain, with towns and cities based on the most well-known locations and capital cities within the UK. Leaders and other major trainers based on British personalities/celebrities, two evil teams so all types are covered by a "major" in-game trainer. Gym Leaders, Elite Four and other major Trainers all have at least one Pokémon capable of Mega Evolution, which may be busted out during a rematch or the first encounter, depending on when you meet them during the story.
Other things of note, you'd get your fossil fairly early on into the game and Eevee, Castform and Porygon would be gifts. The three Mythicals would be post-game stuff with their own challenges and areas. Oh, and you'd meet a mysterious group of people known as the Doctors who have an interest in Dialga and Palkia, though I can't imagine why...
Started basic work on making this a reality using Pokémon Essentials because I'm a huge loser.