>Shikoku-based region between Hoenn and Johto, with Cianwood City being visitable>Chugoku-based region between Hoenn and Johto, with Olivine City being visitable>Tohoku-based region between Kanto and Sinnoh>Super-linear region based on the remaining parts of northern Chubu>>37689155Niigata is a prefecture and city, not a region, but it would probably be the capital city of the last thing I listed.
You have
>Kanto (Pokémon Kanto region)>Chubu (western Kanto, eastern Johto)>Kansai/Kinki (central Johto), designated city Osaka>Shikoku (westernmost Johto)>Kyushu (Hoenn)>Hokkaido (Sinnoh)>Tohoku>Chugoku