The introduction of Pokémon Legends: Arceus takes a big turn from past games, pulling the player from the future to the past through a dimensional time rift. While Pokémon that can manipulate time and space, like the Sinnoh Legendary Dialga and Palkia, have been around since Generation Four, Game Freak has never included any time travel as a major plot element. Because of this, it may seem odd to include it in the new game.
Pokémon Legends: Arceus takes place in the historic Sinnoh region, known as Hisui, long before the modern-day, main series events take place. Hisuian people don't yet live in harmony with Pokémon, with many fearing the power and danger the strange creatures present. This is only compounded by the powerful Alpha Pokémon roaming the wild, and the Noble Legends: Arceus Pokémon cared for by the Diamond and Pearl clans native to Hisui. However, the main character, a skilled Pokémon trainer from the future, seems destined to improve this relationship and bring People and Pokémon together.
While time travel in a Pokémon game may seem like a convenient excuse to bring technology like the Arc phone and other devices to a less technologically advanced setting (which, of course, it is), the choice by Game Freak to include it in Pokémon Legends: Arceus is clever and makes fictional sense. Mythical Pokémon Arceus and the Legendary Pokémon Dialga and Palkia are all capable of bending space and time. Arceus is even known as the "God" Pokémon, with Pokémon history indicating it played a major part in the creation of the world. Because of this, it can be assumed none of these Pokémon exist in a single timeline, and can likely dip into not just the main series timeline as it is known, but other realities alongside the one the games take place in.
Pokémon Legends: Arceus Could Change The Future Within The Games
It is entirely possible that by pulling the main character in Pokémon Legends: Arceus from the future to the past, Arceus has created a divergent timeline and reality. In many sci-fi stories that use time travel to solve a problem, it isn't uncommon for the future to be altered due to the sudden disappearance of a person from their own timeline, and their actions in the past. This butterfly effect could be used to introduce new concepts to future Pokémon games, like clans and Noble Pokémon replacing Gym Leaders, or open-world exploration and Legends: Arceus Pokémon being preserved in a new future.
However, it is also possible the main character's journey to the past could be the reason the Pokémon world of modern games exists. It could be this character's story needed to happen, ensuring Pokémon and people would come together to build partnerships and trainer journeys for hundreds of years to come. Hopefully, more will be revealed by Game Freak about Pokémon Legends: Arceus' effects on the main series timeline, giving players answers on this strange new narrative and adventure.