This game is based on the jpop idol industry, not kpop. I don't see rapping often in jpop.
I think it would be fund to have rapping score, which will be counted by a designated member's Fun and Intelligent Stats, Singing skill does not apply. . Rapping in K pop is the same thing as voice narrations of J pop.
There could be something called "Killing Point". which is during the stage, when centered member is doing signature verses of the song, it is so good, the casual fans notice this parts. Killing Points could be one phrase of a song , dance movements, member's appearance skill+dance skill, or something else.