AH! I read an amazing doujin where her owner was Byakuren!
Holy Gensokyo, do you remember where you found it or what was the name? I NEED to read that!
What if Mystia ran a grilled chicken stand instead?
Well, since it would be a form of cannibalism, we'd have to assume she's gone mad, her insanity caused by the day she was lost in the forest, hungry, and had nothing to eat but sparrows, forcing her to eat her own kind.
She would keep her usual smile and attitude though, selling it as if it was perfectly normal, ignoring people when they'd mention how she's somewhat of a chicken. Soon, she'd get in fights with Mokou, who actually sells the same thing, and would lose, since Mokou is, y'know, an immortal phoenix >_> After her feud with Mokou, she would slowly build up resentment, her hidden insanity causing her childish anger to grow into a full blown out desire to torture and murder Mokou.
She would team up with Kaguya, hoping to bring Mokou pain and suffering. After a few tricks involving Kaguya's plans, she'd accidentally mention her true goal to Kaguya, going much farther than simply ''pissing Mokou off.'' Kaguya would be surprised and try to make Mystia snap out her mad trance, getting in a fight with her as Mystia's remnants of sanity would snap, turning her into a mindless killing machine, which would force Kaguya to kill her.
Thus, Mystia would be no more.
What if Yuuka was challenged by Yuugi for a gool ol' brawl?