DLC for Pokémon Legends: Arceus has been anticipated for some time, and new Hisuian forms should come along with it if it ever happens. Right now, there are 17 Pokémon unique to Hisui counting alternate evolutions, compared to 27 Galarian forms and 18 Alolan forms. Hopefully, Hisui will catch up with Galar if DLC is announced, and more alternate forms will come before the release of Pokémon Scarlet & Violet later in the year.
Most of the alternate forms that have premiered since the concept was introduced to the games have been from Generation 1's Kanto region. Gen 2's Johto and Gen 5's Unova are also well-represented, but the Hoenn, Sinnoh, and Kalos regions have had few if any. With the announcement of Pokémon Scarlet & Violet, it's easy to give up on Pokémon Legends: Arceus DLC, but there's still a chance that it could come, especially since Scarlet & Violet doesn't have a set release date yet and may get pushed back into 2023. New Hisuian forms of classic Pokémon, especially ones from regions that haven't gotten as much attention as others, should come if it's announced before Gen 9's release.
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Hisui eventually becomes the Sinnoh region, so it doesn't seem like Gen 4 variants are likely, but some Hoenn-native Pokémon could be good candidates for new regional forms. Which species would be best is a matter of opinion, and which would make it into Pokémon Legends: Arceus' DLC can't be predicted until news about it actually comes. However, Pokémon from several different games should get variants in Pokémon Legends: Arceus so that there'll be some variety instead of just having all of them be from one region. There are a lot of opportunities with Hisui,
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