[quote=@Rin]

EDIT: My actual suggestion would be to go with Kanto and Johto at least to begin with (for one thing it would mean that dark isn't the only type that's completely absent that way), plus whatever evolutions were retconned onto Pokemon from those regions in later gens, albeit with the caveat that said evolution has to be actually feasible in Kanto (eg. no strong magnetic fields means no Magnezone).

I just think it makes the most sense that way I guess.
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I edited the main post to reflect this as I think it's the best idea.