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.
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, including designs based on traditional Japanese culture and legends.
Drowzee Needs A Hisuian Evolution In Pokémon Legends: Arceus' DLC
Drowzee is a Psychic-Type Pokémon from Gen 1 that evolves into Hypno. It's based on the mythical Baku, a Japanese tapir spirit that devours nightmares. The standard Kanto Drowzee should come to Pokémon Legends: Arceus' DLC, but with a new evolution instead of Hypno. Although both of them eat dreams, Drowzee loses some of its Baku-like qualities when it evolves. A new evolution could lean more into its inspiration and make it feel like more of a mystical spirit. In addition, people are afraid of Pokémon in Legends: Arceus, and since Hypno can control minds, they could be even warier of it than most, and take measures to prevent it from entering their homes at night. Having Drowzee adapt into a different evolution to better feed off people's dreams could be a great backstory for it.
An alternate Drowzee evolution has a great deal of potential in terms of design and stats. The Ghost or Dark Type could be good if it did emphasize the Baku angle, and making it a monotype or a dual Psychic-Type with either both have their appeal. The brown on its lower half could expand to the rest of its body, with perhaps some yellow spots on its back or around its face. Its trunk could grow longer, and it could be slimmer than Drowzee. If new locations come with Pokémon Legends: Arceus' DLC, then a graveyard or other kind of spiritual area could be a good habitat for it. Baku are sometimes depicted surrounded by clouds or fog, and given that Drowzee learns Poison Gas fairly early on, some form of smoke or gas could be incorporated into its design. It could even float on a cloud depending on how far the game takes the spiritual themes.
The Mareep Line Needs A Hisuian Form In Pokémon Legends: Arceus' DLC
Mareep, Flaafy, and Ampharos are Electric-Type Pokémon from the Johto region. They're a popular line, Ampharos in particular, and should have at least one Hisuian form in Pokémon Legends: Arceus' DLC. Having all three gain new forms, just Flaafy and Ampharos, or Ampharos by itself all seem like appealing options. Mareep is a commonly domesticated Pokémon in most media, so it could be interesting to see what they looked like before human intervention. Pokémon Legends: Arceus' empty environments, devoid of people, mean that Mareep would have no humans to herd them or shear their wool, so they could be very different. The Japanese serow sheep or the bighorn sheep could both be a good base.
Regardless of the inspiration, there are a few possibilities for Hisuian Mareep and its evolutions. Rock, Ground, or Fighting could be good secondary Types. More earthy tones instead of bright colors could be a good way to give them camouflage and showcase their survival tactics away from humans. An entirely different direction could be Mareep more based on storms and inclement weather, in which case Dark could be a better second Type and their designs could be based on thunderclouds. Most of Ampharos' Pokédex entries mention that the orb on its tail was used to send signals, so in order for Pokémon Legends: Arceus' to avoid retconning lore, its Hisuian form's appearance should reflect that, perhaps with a bigger orb.
Skitty & Delcatty Should Get Hisuian Forms In Pokémon Legends: Arceus' DLC
Skitty and Delcatty are Normal-Type Pokémon from Gen 3's Hoenn region. They haven't received as much attention as other species, being unavailable in both Gen 7's Alola and Gen 8's Galar without transferring, and should both get new Hisuian forms in Pokémon Legends: Arceus' DLC. Cats are very prominent in Japanese culture and folklore, and since the base game didn't feature any feline Hisuian forms, the DLC should seize the opportunity to introduce one. Skitty are also a domestic Pokémon, so they're another opportunity to show what they were like before they started co-mingling with humans. The traditional Maneki-Neko, or lucky cat, already has a Pokémon in the form of Meowth, as does the Nekomata yokai with Espeon, but the Bakeneko could be a great base.
Depending on the direction the developers take, Skitty could be Pokémon Legends: Arceus' cutest Hisuian form or its most vicious. The Bakeneko is a ghost cat that starts out its life looking like a normal feline but later walks on two legs, so a quadrupedal Skitty and bipedal Delcatty could be good. They're also said to conjure fire, so Ghost/Fire or vice versa is a good option for Its types if the game chooses this direction. They also have long tails, so this could be reflected in their Hisuian form. If Legends: Arceus chooses not to use the Bakeneko, then calico cats, which represent good fortune in Japan, could also be a good inspiration, perhaps with a Normal/Fighting or Normal/Psychic-Type combination.
Pokémon Legends: Arceus DLC Could Give Hisuian Forms To Litwick & Its Evolutions
Litwick and its evolutions, Lampent and Chandelure, are a line of Ghost/Fire Pokémon from Gen 5's Unova. They're based on candles, lamps, and chandeliers respectively, but Hisuian forms could have more traditional appearances, and give the chance for Pokémon Legends: Arceus to show a gentler side of a species that's often been demonized due to its Pokédex entries. Paper lanterns are a very common sight in Japanese festivals, and date back to the 11th century. They could be a great inspiration for Hisuian variants of Litwick and its evolutions. They represent the spirits of those who have passed on and are believed to be protectors against evil, so new forms based on them could be a good reversal of the Litwick line, which has been said to absorb life energy as fuel for their fires.
If the Litwick line gained new Hisuian forms based on paper lanterns, then they could change quite a bit. Litwick could become shorter and smaller, perhaps more like a tea candle than its current form. Lampent could be based on the paper lantern itself, and Chandelure could be a larger lantern surrounded by several smaller candles. Pokémon Legends: Arceus' new evolution requirements mean that it may not evolve with a Dusk Stone, especially if it's given a different Type. Ghost/Grass or perhaps Fire/Grass could be good, and Dark is also an option for either the primary or secondary Type. Pokémon Legends: Arceus' DLC has a lot of opportunities when it comes to Pokémon like Litwick, so it should seize them and give players more Hisuian forms to capture and battle with.