I must do it this way once again. The pack that should save The Sims 4. Because of its complexity, it needs to be an expansion pack.
So, music instruments, songs, music, and bands.
Create new music instruments: bass guitar, flute, strings, saxophone, set of drums, etc, alongside with the existing ones.
Create lots of new complete songs of various genres for lead vocalists. Make the Sims have different voices when singing, depending on their voice type.
Create tons of music parts for rhythm guitar, for bass guitar, for drums, and for all other instruments, that we could combine in an original and unique way with a song.
And the most important, create the tools to synchronize different Sims singing and playing music together, which would make a band.
And when it's all done and you get rich on my idea, I want 10%.
An inspirational video to make you get started.