The Kalos region is so far the only region in Pokemon to receive just one iteration of Pokemon games. Usually, every Pokemon region gets at least three games to its name, and maybe even some updated versions of the two titles that release before them. For example, Kanto, Jhoto, Hoenn, and Sinnoh were all included in at least three games, while Unova and Alola got updated versions of their first two releases. So far, the Galar region only has Pokemon Sword and Shield, but those games at least have DLC updates, which is more than what Kalos currently has going for it.
The idea that Nintendo had plans to create another Pokemon game based out of Kalos has been the topic of rumors and speculation for nearly a decade now, but a recent discovery may have just confirmed that these rumors were indeed true. The source code for Pokemon Sun and Moon recently leaked online, and the data-miners digging through it have found two different placeholder files named "Kalos Reserved 3" and "Kalos Reserved 4," suggesting that Nintendo planned to give Kalos two more games.
The existence of the files themselves doesn't offer up much information regarding what the two potential games would have been. However, the mentioned pattern followed by the other regions does make it possible to reliably speculate that Nintendo was planning a "Pokemon X and Y 2" of some sort. Pokemon Black, White, Sun, and Moon all received sequels or enhanced versions of themselves. Since Pokemon X and Y came out right between these two sets of games, it makes sense that they would follow this pattern instead of receiving something like "Pokemon Z."
Maybe Nintendo will fall back on these unused Kalos games in the future. All sorts of rumors are going around that a thrid DLC for Pokemon Sword and Shield will release after Crown Tundra, and that DLC will focus on a return to the Kalos region. They're just rumors, of course, so just like this datamine, they should be taken with a grain a salt, but Kalos fans would love to see the region make a comeback nonetheless.
Overall, 2020 has not been a good year for Nintendo when it comes to source code leaks. Several of them have come out revolving around the company's games and consoles. Pokemon leaks have been happening for years, but Pokemon Sun and Moon only came out four years ago, meaning leaks are starting to happen for Nintendo's more modern properties. Hopefully, the company will find a way to plug things up soon before something more sensitive gets revealed.