Like, minecraft maps already divide the world into 128x128 blocks (or 8x8 chunks) large squares, so adjecant maps align perfectly, so what if each of those areas had a pokemon region assigned to them? On a map section that is designated as "kanto", only kanto pokemon spawn, on a "johto" themed section of the world, only johto pokemon spawn.
This would also allow for regional forms to use the actual game mechanics, if each pokemon remembered the area, it was originally caught in. Like how some pokemon can the gigantamax factor, there could be a genetic trait for each region, which can be passed down via breeding, depending on wether an everstone is held by one of the parents. The actual games keep track of where it was caught, or in case of event pokemon its listed as "fatefull encounter"
Also could shellos/gastrodons forms be based on the x-coordinates? like if it spawns in an area with an positive x-axis, its blue, with an negative x-axis its pink