They're not likely to bring the Singing skill to another pack, and I doubt we'll see TS3-style Bands, but Clubs have it covered. All we need are more instruments and the ability to sync up the instrument/singing (a "play together" interaction), and we'd basically have everything we need.
The "play together" would probably have to be a pre-pack patch (due to the Singing skill), but some more instrument and some rockstar-appropriate outfits could make for a decent Stuff Pack.