Everyone has a favorite Pokémon game, and similarly, everyone has a favorite Pokémon generation. To make each game feel fresh and unique, the developers have tried to build upon the previous entry. In each new game, there are new mechanics, features, and, of course, regions and Pokémon. Naturally, some mechanics and features have carried over to other games in the series, such as hatching eggs and the Exp Share item. However, many other aspects of the Pokémon games have only been featured in one game. For example, Generation II allowed players to explore the Kanto region from the first game once they had completed the Johto region.
In Pokémon Gold and Silver, players battle their way through the Johto region and eventually compete in the Pokémon League, battling against the Elite Four. For most Pokémon games, that would be the end of forward progression for players, but this is not the case for Gold and Silver. After finishing off the Elite Four and the Pokémon Champion in those games, the entire Kanto region from Pokémon Red and Blue is unlocked for players to explore. Players can battle each of the gyms from the Kanto region, challenge that region's Pokémon League and the Elite Four, and then face off against Red from Red and Blue. Oddly enough, this never reappeared in subsequent Pokémon games, but it would be a perfect addition for a Pokémon Legends: Arceus sequel.
Pokémon Legends: Arceus is set in the early days of the world of Pokémon. In Arceus, players explore the Hisui region long before it became the Sinnoh region. Players act as scouts and researchers for the Galaxy Team, and they research, scout, and discover new Pokémon. While there has not been a sequel officially announced, it would presumably take place in an entirely new region that players would have to discover. Naturally, players explore the region to capture Pokémon, but since this game takes place centuries before the discovery of several regions, there are presumably still some regions or areas that have not been fully explored. Opening up a brand-new region after players are finished with the base region would only add to the mystery and discovery.
Not only will this make players feel like explorers and scouts, but it will also further expand the lore of Pokémon. Of course, Pokémon has never had overly deep lore, and it certainly isn't known for its story. However, it does have a ton of story threads and memorable characters that are often connected in some way throughout the series. Some of those threads and connections can be seen in Arceus. By adding in two new regions and setting the game in the past, the developers will be able to give reason and meaning for the technology, culture, and philosophies of future games.
Adding in a second region to a Pokémon Legends sequel would also provide developers with the opportunity to expand on the Noble Pokémon from Legends and their battles or introduce early versions of gym leaders and gym battles that appear in the mainline games. If gym battles are included, players might meet some ancestors of gym leaders from the other Pokémon games. The extra region would also increase the number of regional variants of Pokémon that have become a staple of the series. Obviously, there would be regional variants of Pokémon that haven't been seen, but it would also allow players to see ancient variants that have changed as the region changed.
It is unclear if there will ever be another entry in the Pokémon Legends series. The next Pokémon games, Pokémon Scarlet and Violet, are entries in the main series. While Pokémon Legends: Arceus was a major hit, Nintendo has a history of not following up on some games if the studio doesn't feel like there is anything new or inventive to add. However, with the immense success and new life that Arceus breathed into the series, it would make sense to follow up on it. Plus, some of the most obscure games in the Pokémon series have received sequels.