Probably the closest you'll get:Unrelated to any current conversations but... I just stumbled on these and it got me thinkin'. Coulda been a good song collab partner for Calli before she got too serious about the music stuff. Not that her current rap game is anything too serious but yeah...
IIRC she has said that she wants to collab with Enna and Reimu, but I'd guess that a song would require them to overcome an insane amount of distilled Japanese managerial autism.