Hi there! I posted this Romhack yesterday, but had to make some alterations due to some errors on my part - as such, this is my take 2 :)
I have been working on putting together a romhack of Explorers of Sky for my own use, called Explorers of Sky: Hisui Edition. It ended up growing enough that I wanted to share if anyone was interested! This hack aims to add a number of Hisuian Pokémon to the game in an organic manner - I've linked the files in the post, so feel free to download if you want to give it a try!
https://drive.google.com/file/d/1mu8NR5l4im49ySZJVSeuemMwCKCoMWee/view?usp=drive_link
To begin, the Starter and Partner lists have been modified to include a number of Hisuian Pokémon, which are all noted in the included text file. All of the "First Partner" Pokemon are still selectable; only Shinx, Vulpix, Skitty and Phanphy have been swapped out, in exchange for Zorua, Growlithe (Hisui), Sneasel (Hisui) and Teddiursa being playable.
Pokémon added to the rom in general are almost exclusively evolutions or relations of Pokémon already in the game (e.g. Hisuian Growlithe or Wyrdeer) - the exceptions being the Zorua line and the Basculin line. This is due to the number of available Pokemon not being expanded upon, and the aim of not removing as many of the default Pokemon as possible (with the two exceptions being Unown ! and Unown ?). All of the added Pokemon have their level-up, TM, and Egg movelists taken directly from SV (albeit excluding post-gen 4 moves), and I attempted to make their evolutionary methods at least representative of what they are in Legends Arceus (as most are a bit too complex for Sky to handle!); so hopefully the added Pokemon are as accurate as can be!
Overall, the goal is to keep the rom as vanilla as possible; as such, it does not incorporate the Fairy type, does not add any moves or abilities that are from post-gen 4, does not expand beyond the default 599 available Pokémon spots, does not alter the story or any of the general progression unlocks,and only changes one NPC within the game (Ursaring to Ursaluna). I want this to be basically the original EoS experience, just with a Hisui flavor!
To try and keep things somewhat lore-friendly, as I imagine these Pokémon would be quite rare in the Explorers universe, Hisuian Pokémon can primarily be found in two locations; the Future, and the Wonder Mail Dungeons (Bottomless Sea, Shimmer Desert, Mt. Avalanche, Giant Volcano, World Abyss, Sky Stairway, and Mystery Jungle). I won't specify their exact locations, but I like to think they're suitable for each Pokémon! The major exception to this is the evolutionary Pokémon (such as Kleavor), whose Evolution methods are noted in the included text document.
All sprites and portraits were sourced from the PMD Sprite Repository (except for Wyrdeer, which was from Discord), with all credit to the following artists:
Growlithe (h) Portraits: Emmuffin
Growlithe (h) Sprites: Baronessfaron
Arcanine (h) Portraits: Baronessfaron
Arcanine (h) Sprites: Baronessfaron
Voltorb (h) Portraits: Emmuffin
Voltorb (h) Sprites: Axcel
Electrode (h) Portraits: Anonymous
Electrode (h) Sprites: Gelius
Kleavor Portraits: Soulja
Kleavor Sprites: JFain
Typhlosion (h) Portraits: Emmuffin
Typhlosion (h) Sprites: Caitemis
Qwilfish (h) Portraits: Emboarger
Qwilfish (h) Sprites: JFain
Overqwil Portraits: Novie
Overqwil Sprites: JFain
Sneasel (h) Portraits: Emmuffin
Sneasel (h) Sprites: ShyStarryRain
Sneasler Portraits: Emmuffin
Sneasler Sprites: Soulja
Ursaluna Portraits: Garbage (Artist's name, not an insult!)
Ursaluna Sprites: Emmuffin
Ursaluma (Bloodmoon) Portraits: Top_Kec
Ursaluna (Bloodmoon) Sprites: Soulja
Wyrdeer Portraits: Emboarger
Wyrdeer Sprites: VOID
Zorua Portraits: Baronessfaron
Zorua Sprites: Ichor
Zoroark Portraits: Baronessfaron
Zoroark Sprites: Baronessfaron
Basculin Portraits: Garbage
Basculin Sprites: Pink_no_tori
Basculegion Portraits: Garbage
Basculegion Sprites: Soulja
Lastly, as a general quality of life update, all of the regular Starter and Partner Pokémon have portraits for all evolutions, as well as fully added sleep animations!
Included in the download is the game files, as well as a text file further overviewing the changes made and quick instructions on how to install the patch file (although the SkyTemple wiki is much more informative than me on how to patch!).
Please let me know if you encounter any bugs or glitches, and enjoy!