The world of Pokemon features several regions scattered across the globe, with a new one being added with every generation so far. While each new entry in the Pokemon franchise is seen as a standalone experience, there are several regions that are closely related. Some regions are off in their own corner of the world, but others are next-door neighbors.
While there has only been one set of games in the entire series that allows players to travel between regions, the lore of Pokemon shows that the world is much more connected than many players realize. In fact, there are various crossovers to discover just under the surface.
A Larger Country
In Pokemon Diamond and Pearl, it is explained that Sinnoh, Kanto, Hoenn, and Johto are part of one nation, with Kanto and Johto being the most connected so far. In Pokemon Gold and Silver, once players defeat the Johto Elite Four and Champion, they are allowed to cross through the mountain that separate Johto and Kanto via Tohjo Falls and continue exploring the Kanto region. This is the biggest connection between two regions seen in the Pokemon series to date, and why so many fans loved Generation 2 - leaving them hopeful for a new remake of that generation.
Sinnoh and Hoenn are also a part of this larger country, which at the time of writing still is unnamed. Hoenn and Sinnoh are based off the Kyushu and Hokkaido regions of Japan, respectively. These are more off-set from Kanto and Johto (with Kanto based on the real-world Kanto region, and Johto based on the Kansai region). Since Pokemon Legends: Arceus takes place in an ancient version of the Sinnoh Region (at that time called the Hisui Region), perhaps players will learn more about its geographical location compared to the other regions.
Outside of the major landmass and surrounding islands that form Pokemon's major country, there are also some other references to connection regions in the isolated regions. In terms of connecting structures, there are the Sinjoh Ruins. The actual geographical location of the Sinjoh Ruins is unknown, and its name comes from the fact that two researchers (one from Sinnoh and one from Johto) worked together to create the temple there. The aesthetics of the ruins combine the Ruins of Alph of Johto and the Spear Pillar of Sinnoh, but outside of aesthetics and name, it's not known how (if at all) these ruins bridge the Sinnoh and Johto regions. As the temple was built in honor of Arceus, perhaps there may be some reference to it in Pokemon Legends: Arceus, as the game will hopefully explore the regions ties to Pokemon mythology.
The Isolated Regions
While those four regions are part of one major country, there are still several other regions from the mainline games that are isolated: Unova, Kalos, Alola, and Galar. Much like the other four regions were based on Japan, these are based on other parts of the world, and are likely further away from the main country because of this.
Unova was the first "far-off" region introduced in Pokemon Black and White. Unova is based on the state of New York in the United States in general, with Castelia City being based on New York City. There is a myserious cave in Unova called the Cave of Being which has some connection to the Sinnoh Region, but the games do not elaborate any further. Additionally, nearly all of the recently announced Hisui Forms for Pokemon Legends: Arceus, as well as some of the new evolutions, are derived from Unovan Pokemon, which could hint at a bigger connection between these two regions.
Kalos was introduced in Pokemon X and Y, and is based on Northern France. The region itself is considerably far away from the main country if the Pokemon anime is to be believed, as characters needed to travel by airplane to return to other regions. If the geography of the Pokemon world is accurate to real life, the Galar Region would likely be its next closest neighbor, though nothing is explicitly stated in-game.
Alola was introduced in Pokemon Sun and Moon, and is based on Hawai'i. Despite having a large number of Kantonian Pokemon reprised in Alolan Forms, it's not clear how close Alola is to the Kanto region, or any other region for that matter. Being a string of islands in the middle of the ocean, it is unclear where Alola truly is in relation to the rest of the world.
Galar is the most recent region to be introduced, coming in Pokemon Sword and Shield. This region is based on the United Kingdom, specifically England, with the Isle of Armor being based on the Isle of Man and the Crown Tundra being based on Scotland. Theoretically, Galar may be closer to Kalos than any other region given England and France are near one another, though nothing is set in stone. Many fans speculated that a Kalos DLC would be released for Pokemon Sword and Shield to tie the two regions together, but those rumors amounted to nothing in the end.
Players are getting ready to revisit the Sinnoh region with the release of Pokemon Brilliant Diamond and Shining Pearl, and then they will get to see a brand-new take on the Sinnoh Region when Pokemon Legends: Arceus launches next year. While it would be surprising to see any new information come out of the Diamond and Pearl remakes, Pokemon Legends: Arceus has a lot of potential to give players new lore about the Hisui/Sinnoh Region and its connection to the Pokemon world at large. There is still some time yet before fans get to dive into the ancient version of the Sinnoh region.
Pokemon Brilliant Diamond and Shining Pearl release on November 19, 2021 for the Nintendo Switch.