In terms of characters who aren't available to play, but are potentially useable due to the fact that this game has already made use of time-lost characters/side characters, we do have some interesting slots.
Liyue still has Guizhong, just as an initial observation. May be a potential answer to the lack of geo characters.
Inazuma has Raidens lost friends/family, that being Makoto, Kitsune Saigyuu, and whatever the name of their oni friend was:
View attachment 4780425
Sumeru has Jeht and Rana, Babel, as well as the as of yet unseen goddess of flowers, so you can put whoever you like in that slot like alice, alongside Idyia from the summer event that took place in the desert.
View attachment 4780454
View attachment 4780455
View attachment 4780460
View attachment 4780514
For Fontaine, we have a few extra options. Mary-Ann, Focalors, Could potentially cook up something with Egeria if you want to do the custom route (She's a blank just like the flower goddess):
View attachment 4780505
As for Natlan...Gonna be honest, Natlan doesn't have a whole heck of a lot of extra characters of note, at least not female ones.
Bona, perhaps?
View attachment 4780527
All in all, I'm not overtly concerned about lack of bodies. Also remember that these are
in addition to what we already have, so breeding is going to get crazy as time goes on. And who knows, perhaps, by the end...
View attachment 4780539
Here is my early list of possible special NPCs that haven't been implemented.
(I have moved Zhiqiong, Harvia (now possessing Wanyan), Fujin to special NPCs.)
Mondstadt:
Sansa (not sure what a bard can do)
Liyue:
Guizhong (main heroine, need to find a way to resurrect her)
Bonanus (need to find a way to resurrect her)
Indarias (need to find a way to resurrect her)
Lingyuan/Changsheng (Don't know if they could get official human forms)
Inazuma:
Raiden Makoto (main heroine, need to find a way to resurrect her)
Kitsune Saiguu (Hanachirusato)
Mikoshi Chiyo (need to find a way to resurrect her)
Asase Hibiki (somehow possessing Neko or Etsuko)
Kano Nana, Miyuki and other shrine maidens (I guess Nana belongs to Shuumatsuban?)
Hanyuuda Chizuru
Hiiragi Chisato
Sango
Wakamurasaki (omni-ubiquity net)
Ogura Mio (possible outfit system (also Chiori))
Hanayama Kaoru (possessed by Tatarigami)
Julie, Parvaneh (cooking system (also Verr Goldet/Xiangling/Escoffier))
Tsumi (final boss of Enkanomiya)
Lyudochka(disguised electric cicin)
Tae(disguised cryo cicin)
Sumaru:
Dunyarzad
Rana
A lot of scholars and elite scholars (alchemy/potion system (also Lisa/Sucrose/Emilie))
Idyia (oceanid)
Tadhla
Jeht
Babel (possessed by Nabu Malikata)
(need some one to train Eremite Floral Ring-Dancer)
Fontaine
Egeria (main heroine, just need to find a way to resurrect her)
Estelle, Sevigne (forge system)
Yseut, Deianeira (local legend)
Ruthless Calla Lily
Mary-Ann (oceanid)
Erinnyes (need more information)
Clervie
Melusines
Natlan
Amina
Chuychu
Huitzilin (need to find a way to resurrect her)
Hine (need to find a way to resurrect her)
Bona (need to find a way to resurrect her)
Wayward Hermetic Spiritspeaker
Yohualtecuhtin(maybe somehow in the future)
A lot of tribal warriors
Some local legends
Snezhnaya
Signora (main heroine, will be implemented in Inazuma version)
Columbina (main heroine)
Arlecchino (will be implemented in Fontaine version)
Sandrone (more information needed, not sure if she is a doll controlled by some man
Alain.)
Tsaritsa (main heroine)
Scarlett
Other
Skirk (main heroine)
Vennessa (need more information)
Rhinedottir (need more information)
Nicole Reeyn (need more information)
Final Stage
Istaroth (Presumably Paimon)
Ronova
Heavenly Principles
*Kujirai Momiji/Loumelat/Delaroche/Oroluk (Fishing system is weird in this game)
*I haven't designed who can be the boss of Sea of Bygone Eras/Ancient Sacred Mountain