Id like a great background vocals library. I dont like any of the available ones. Something along the lines of the new guitar libraries from NI would be great.
Many variations is needed, different voices, male/female, young/mature, different timbres ect, so it can be blended and made unique sounding. A wide repetoire of simple phrases would be good, as they can be mangled, holiday phrases, seasonal phrases, american english, uk english, spanish, other languages.
Maybe something for different genres, that could be added as packs, as well as continuous additions of singers, a la the Toontrack midi concept, just with audio phrases.
No lead vocal, as I would never use a library for lead vocals. Just a solid backing library, with great sound, flexibility and options. That would be HUGE!