What I was thinking about was a simple rom hack from fire red where all latest moves and higher generation features have been implemented like physical/special split & fairy type. I would skip the exp system tho.
The story could be based arround pokémons getting imported from other regions and these pokémons have adapt themself to Kanto forms (like Alola). Some examples:
Gym leaders could have imported pokémon from different regions to fight you with. Brock could have an Aloha Geodude for example.
As far the story goes, it could be based arround all this pokémon importing & how it effect pokémon. Kanto has implemented a rule that it's harder for people to import exotic pokémon. Gym leaders & high rank officials have less restrictions regarding this. Their could be two groups in this game. One group that want to remove all exotic pokémon from Kanto and one group that want free borders.