It would follow Gen III in the fashion that Hoenn is based on the Kyuushuu-Okinawa region.
Talking about the geographical data of Japan, Chuugoku-Shikoku is a little bigger in area, but a little less populated than Kyuushuu. And it's a lot smaller in area, but a lot more more populated than Hokkaidou (which Sinnoh is based on).
Here is how it would fit:
The Chuugoku region is often separated this way, in a reference of the Yin/Yang:
- The San'in region, the northern or shady side of the Chuugoku mountain range
It consists of the Shimane, Tottori and the north part of Yamaguchi prefectures.
- The San'yo region, the southern and sunny side of the mountains.
It consists of Okayama, Hiroshima and Yamaguchi prefectures.
The Yin/Yang itself could be a possible reference of the Black & White of the game titles, as it represents a balance between seemingly contrary forces and describes how they are interconnected and interdependent in the natural world, and how they give rise to each other in turn. It's NOT a good vs evil conflict. And it's represented by this symbol, the Taijitu:
There’s a screen in which we can see a two-tiered bridge. There is a very famous two-tiered bridge in the Chuugoku-Shikoku region, the Seto Bridge, that is 13.1km (or 8.1 miles) long:
Home of the Izumo-taisha in Izumo, near the capital Matsue. It is one of the most ancient and important shrines (considered to be the 2nd most important) in Japan and where Amaterasu, the goddess of the sun and one of the principal Shinto deities, is enshrined.
Susanoo, the god of sea and storms, the brother of Amaterasu who slayed the dragon Yamato no Orochi in japanese mithology, is also enshrined close to Matsue, in Yakumo.
The capital, Matsue, sits between Lake Shinji (which curiously is the name of Lake Verity in the japanese version) and Nakaumi - a salt water lake with a volcanic island, the Daikon Island, with lave tubes - along the banks of the Ohashi river connecting the two. Due to the prominence of the lakes, the river and canals in the city-scape and scenery it is sometimes called the "water city".
In the city of Ota, there was the Iwami Ginzan, a silver mine, and nowadays it’s a World Heritage Site.
There is also an archipelago on the coast of Shimane called the Oki Islands.
A big and important shrine should obviously be on the game and maybe using the mythology of Amaterasu or Susanoo and Orochi as a part of the game.
I’d like to see some sort of Orochi legendary. Maybe a Psychic-type Gym could fit the city, as of the importance of the Shinto shrines in this area.
The silver mine could be used as a mine/cave dungeon.
Near Tottori city, capital of the prefecture, there are the Tottori Sand Dunes, considered to be the only desert in Japan. The prefecture coast is famous for the bluefin tuna. Also, the highest mountain in Chuugoku is in this prefecture, the Daisen, which is a volcanic mountain.
Bordering the prefecture and the second highest of the Chuugoku Mountains is the Mount Hyono (which means literally Ice Mountain).
A desert area near a city with a Ground-type Gym.
The Daisen could be the Victory Road, while Mount Hyono could be an Ice-type cave.
It is home to the Akiyoshi Plateau, where you can find the Akiyoshi Cave, a wide, spacious and full of passages. A common visiting place.
In Iwakuni city, there's a beautiful bridge with five linked wooden arches leading to a castle. Kintai Bridge is one of the Japan's three famous bridges.
The immediate surroundings are noted for their cherry blossom and it's one of the most popular tourist destinations in Japan.
North of Yamaguchi prefecture, on the Japan Sea, stands Oumijima, known as the Sea Alps, as there are sharp stone columns thrusting out of the ocean and sheer cliffs with ocean caves.
The Yamaguchi prefecture is really close to the Kyuushuu region (Hoenn’s real world counterpart) and it is only separated by a stretch of water called the Kanmon Straits. On the Chuugoku side of the straits is the city of Shimonoseki.
Accessible via ferry from Shimonoseki stands Ganryuujima, an island famous for the duel between Miyamoto Musashi and Sasaki Kojirou (the samurai that the Japanese names of Jessie and James – Musashi and Kojirou – are based on).
The Akiyoshi plateau is a possible place for a Safari Zone in a game. And there should be a cave area.
A city based on Shimonoseki, with a tall tower or lighthouse, could lead on lots of connections with Hoenn and possibly hint at a future remake of Gen III games.
Hiroshima city is the capital of the prefecture. A world famous city for being the target of the first atomic bomb during World War II. It's a metropolis, quickly and strongly rebuilt after the bomb incident, but also has museums and ruins, like the Genbaku Dome in the Peace Memorial Park, left to show the scars of the past.
The city is often linked with the Yakuza (Japanese mafia), thanks to many famous yakuza movies taking place in Hiroshima.
The Itsukushima shrine on the island of Miyajima stands in the Inland Sea, really close to Hiroshima, and the tides makes it looks like it’s floating sometimes.
Hiroshima is a very likely candidate for being the city represented in the screen with the big city surrounded with water.
The historic ruins in Hiroshima could be a place where you find Ghost Pokémon. Also, as it is as big city and often linked to the yakuza, so it’s a good guess for the headquarters of the evil team, or even to have the awaited Dark-type Gym.
There should be an area based on Miyajima, with Stantler in it, as there are lots of deers roaming freely through the island.
The capital of the Okayama prefecture is Okayama city, linked through the Marine Liner, the train that rides on the Seto Bridge aforementioned, to Takamatsu, the capital of Kagawa, in Shikoku.
It's famous as the setting of the japanese fable Momotarou, about a boy that appeared from within a peach and later would befriend a dog, a pheasant and a monkey on a quest to the Oni (demon/ogre) island. This island is associated with Megijima, an Inland Sea island near Takamatsu. The caves at the top of the island is said to be where the Oni lived.
I could see some Pecha Berry trees around here.
The Seto Bridge linking the two landmasses could be a Cycling Road or something similar.
I think it is an interesting idea to have a Fire-type Gym, with a gym leader with a name like Momoko/Momoka (as momo is peach in Japanese) having a Torchic, Monferno and Houndoom (to reference the Pheasant, Monkey and Dog from the fable).
The capital, Takamatsu, is the one mentioned above. It is where one of the most famous and most beautiful historical parks in Japan can be found, the Ritsurin Park.
As for the island near Takamatsu mentioned before, there could be a cave and maybe some event in which you should take 3 specific Pokémon in your party. Could that be the use for the shiny beasts from Movie 13?
The park could be the forest you run into early in the game.
Famous for its citric fruits and responsible for a substantial fraction of Shikoku's electricity, specially from Ikata's nuclear power plant and wind farms.
The capital, Matsuyama, is famous for the hot springs, of which there is the Dogo Onsen, the oldest hot spring bath house in Japan.
A city with a nearby power plant filled with Electric-type Pokémon.
The prefecture has a generally small commercial scale and its industries are generally based upon utilizing the natural resources found within the prefecture.
Sakamoto Ryoma is a really famous samurai born in Kochi and was a leader of the movement to overthrow the Tokugawa shogunate during the Bakumatsu period in Japan. A Meiji Restoration hero.
On the coast village of Usa, in Kosa, stands the world famous Usa Marine Biological Institute, one of the oldest and largest marine biology research centers in Japan.
The starting town could be around here on the southern tip of the region, being the home of the protagonist and the professor ‘s lab.
The Iya Valley is a scenic area known for its dramatic mountain valleys and old vine bridges. There lies the second-highest peak on Shikoku, the Mount Tsurugi.
Cianwood City in Johto is based on Naruto, a city on the northeastern tip of Shikoku that is part of the Tokushima prefecture.
So, this is how I picture the region. Remember, this is SPECULATION:
I’m sorry that it turned pretty big, I just noticed now how long it became.