Like Pokémon Diamond and Pearl, and the remakes announced today, the game is set in the Sinnoh region. The game should function as a kind of prequel.
The game should offer an open world. According to the trailer, people from all over the world have traveled to Sinnoh to build a new city. The goal is to create the very first Pokédex. Players can throw Poké Balls and fight with Pokémon. The starters are Rowlet, Cybdaquill and Oshawott. Logically, the legendary pokémon Arceus should also play an important role.
It is not the first time that open world elements have appeared in Pokémon games. In Pokémon Sword and Shield, there was already an area to visit that was completely open and free. However, it appears that Legends: Arceus is completely open world.
Pokémon Legends: Arceus is due to release on Nintendo Switch sometime in early 2022.