I like the idea that some pokemon are introduced in games where they’re not in their ‘home’ region.
Like, for example, Yanma. Introduced in Johto. Everyone thinks of it as a Johto pokemon. But it was incredibly rare and usually only found when it started swarming.
But in Pastoria’s Great Marsh, it’s fairly common. So maybe Yanma are native to Sinnoh, and they only very rarely show up anywhere else, and really they’re Sinnoh pokemon despite us thinking of them as Johto pokemon.
Or like, Eevee. Which has to be found as a one-off gift in basically every region… except Kalos, where they’re found freely in the wild. So Eevees come from Kalos, and back in Red/Blue you found a rare imported one.