I accidentally voted Siegfried just cause I main Siegfried, but Sophitia probably has a better fighting style to carry her own game.
Her new timeline story is also quite interesting all be it quite Mary Sue.
Siegfried's story could also be interesting.
1st Game - Goody two shoes knight, father gets murdered you need strength to exact revenge so you start your journey to find the Soul Edge, eventually find it maybe midway through the game, but start going mad unlocking new moves and shit....turn into to Nightmare.
2nd Game - Nightmare defeated by your old friends at the start, you are back to being Siegfried.
No powers because videogame reasons, find Soul Calibur start building up powers again hunting down your Soul Edge possessed Armor (Nightmare).
I could see Scorpion in a Sekiro/Souls-type game set in the MK universe. Taki would also make sense in a Nioh/Souls game since she is a demon hunter(?).
Kyo Kusanagi (King of Fighters), Sub-Zero (Mortal Kombat), and Nina Williams (Tekken) had their own spin offs, but were below average. There are already plenty of characters in fighting games originating from standalone titles like Ryu Hayabusa (Ninja Gaiden), Ralf/Clark (Ikari Warriors), Athena (Athena), etc. There are no need for more.
How about a Yakuza-like game with Ryu set in a comfy japanese village and the forest and temple where he used to train with Ken? That'd be cool imo, could serve as a prequel of the main series.
I would love, LOVE a Chun Li character action game, or a modernized beat em up rather, by Platinum, by Capcom themselves... sexy, iconic, great moves, don't know why it hasn't happened already. Not to mention she's a policewoman, taking down criminal baddies through gritty street levels would be a natural fit for her standalone game. Something like Sifu, but flashier: