Hi, thanks for playing!
Unfortunately, most of the other characters' movesets are a mere fraction of Kiriko's, so it's a ton of work to bring them up to the standard of a playable character. The taunt system does open up the possibility of bringing in some of their moves, but this is something that I have yet to properly design around.
If I were to make the other bosses playable characters, they would probably be in separate games dedicated to them rather than being part of this one, although there's no guarantee it'll even be of a similar genre (let alone if I'll even get around to making it...).