I mean, our DEV took the time to make her character good-looking and likeable. A spin-off with her wouldn't work, since she is too chill. The whole point of Yuna is that she's like a tsundere so you have to build your relationship with her from the ground up while secretly doing the recording thing
Kaede is too chill. Maybe an in-game side mission where you have sex with her could happen, though.