The new Pokémon that will appear in Pokémon Legends: Arceus could be considered part of the upcoming Pokémon Gen 9. Despite being a spinoff game, the Hisui region of Legends: Arceus is home to a number of never-before-seen Pokémon with interesting designs and backgrounds. Their inclusion to Legends: Arceus could indicate that these new Pokémon will officially be added to the mainline series in Gen 9.
The new Pokémon introduced in Pokémon Legends: Arceus come in the form of evolutions and regional variants of established Pokémon. A leak regarding how many Hisuian forms Pokémon Legends: Arceus will have suggests the game will introduce 17 in total, and three new evolutions like Wyrdeer and Kleavor have been officially announced. More than likely, the effort put into creating such a large quantity of detailed Pokémon is to be a preview into some of the newcomers fans can look forward to in the eventual Gen 9 titles.
There are numerous instances of Pokémon being revealed before the generation to which they officially belong. Misty's Togepi first appeared in an episode of the Pokémon anime in 1998, over a year before the Japanese release of Pokémon Gold and Silver. Similarly, Lugia was featured on the promotional art for the movie Pokémon the Movie 2000: The Power of One prior to its official reveal as a Gen 2 Pokémon. Given how the recent Pokémon additions shown are given the important title of Noble Pokémon in Pokémon Legends: Arceus, it could be that they are being given similar treatment to prepare them for officially being part of Gen 9 mainline games.
Pokémon BDSP Sets Up New Legends: Arceus Pokémon For Gen 9
The fact that none of the new Pokémon revealed in Pokémon Legends: Arceus appear in Pokémon Brilliant Diamond and Shining Pearl hints that they are being saved for Gen 9. Legends: Arceus' Hisui region shares a historical connection to BDSP's Sinnoh region, leaving open the possibility of the ancient Pokémon to appear in the remakes. However, the new evolutions and variants are absent in BDSP, as are other Pokémon introduced after Gen 4. While future updates to BDSP could theoretically add them to the game, it seems more likely that BDSP is not the generation Legends: Arceus' new Pokémon are planned for.
Still, BDSP could be used as another platform to promote Legends: Arceus' new Pokémon. Players have discovered a book in BDSP suggesting a Hisuian Qwilfish in Pokémon Legends: Arceus, since it was not present in the original Diamond and Pearl. If the new Pokémon from Legends: Arceus are to be officially part of Gen 9, using BDSP to create intrigue around them would be a useful strategy in promoting Gen 9's new additions.
Pokémon Mystery Dungeon Revealed New Pokémon Before Their Generation
The tactic of revealing Pokémon in a spinoff game before their generation was previously employed in Pokémon Mystery Dungeon: Red and Blue Rescue Team. Released in 2005, these titles featured Pokémon from the first three generations of games. However, the games feature statues of the Pokémon Lucario, Bonsly, Mime Jr. and Weavile, all of which would officially become part of Gen 4 in 2006. Lucario is further made important, with its name being applied to the game's highest rescue team rank.
In the same way, the new Pokémon of Pokémon Legends: Arceus could be meant to prepare players for an upcoming Gen 9 appearance. The Noble Pokémon Kleavor of Legends: Arceus, for example, has significance in the game despite having never appeared in the series mainline. Much like the Gen 4 Pokémon in Mystery Dungeon, the new Pokémon of Legends: Arceus have enough detail put into their designs and implementation to suggest that they will be reappearing again. Gen 9 can make Pokémon like Kleavor normally obtainable similar to how Lucario became a normally-obtainable Pokémon in Gen 4 after Mystery Dungeon.
Gen 9's Region Could Be Suitable For Legends: Arceus' Ancient Pokémon
If the new Pokémon of Pokémon Legends: Arceus are considered part of Gen 9, it could mean the next region will be one suitable for the return of ancient Pokémon. Regions like Sinnoh from BDSP do not seem to have the conditions necessary to reproduce the Pokémon that existed in Hisui, but Gen 9 could take place in a region that fulfills those environmental conditions, leading to the reappearance of the new evolutions and variants. Many real-world countries could inspire Pokémon Gen 9's region and achieve this, creating a new historical period for these Pokémon to reside.
Pokémon Legends: Arceus' new Pokémon existing at all in Gen 9 could give the new region a much different feel than previous games. Previous regions have varied in societal advancement; Sinnoh's Celestic Town has a traditional, rustic feel, while Unova's Opelucid City has futuristic architecture. For ancient Pokémon like those in Legends: Arceus to exist in Gen 9, the society of the game could be old like Hisui, featuring the Pokémon during an age where they still existed. Alternatively, Gen 9's region could be the most technologically advanced the series has seen to date, restoring Legends: Arceus' Hisuian forms as fossil Pokémon to the point where they have become abundant in the region once more.
While the possibility exists that the new Pokémon introduced in Pokémon Legends: Arceus will not appear in the mainline series, it seems more likely that they will be officially part of the Pokédex in Gen 9. Including not only new Pokémon variants but new evolutions to existing Pokémon seems the perfect opportunity to advertise the next generation of Pokémon games. Given the series history of hinting at and previewing Pokémon before their official generation, Legends: Arceus could very well be following in the footsteps of Pokémon Mystery Dungeon with its reveals, allowing players to gain familiarity with what is to come. Possibly, the addition of these Pokémon could be setting up Pokémon Gen 9 to be open world like Legends: Arceus, giving trainers the functional use the new Pokémon in Legends: Arceus provide instead of just including them normally. No matter how Gen 9 implements the new Pokémon revealed in Pokémon Legends: Arceus, showing a preview helps increase excitement both for the upcoming game and for the series as a whole.