Rock Bands yes, but not just limited to this genre. I believe there could be a GP or even EP dedicated to creating different kinds of bands through something similar like the club system in GT. Depending on the musical instrument skill level, a chamber orchestra (violins and cellos) or a kids choir/ gospel choir (Vocals and piano/ organ) can be formed and with a high enough ‘band’ skill level, live gigs can be organized.
Drums, Electric and Bass guitars are sorely missed as basic instruments. I do appreciate the ability for a Sim to sing along with guitar or piano and hope this will be developed more.
I believe developing Immersive band/music gameplay can be quite challenging: recording all the individual instrumental tracks at different Skill levels and adding animation for each different instrument and as a band must take a chunk of Resources, but if there is a high enough demand for it I can see a great pack coming out of this.
Bands were mentioned in the latest survey, so there is scope for it if enough players are interested.