I'd like to see extra articulations in the Vienna Choir such as humming (mmm), and at some point I would really like VSL to address the lack of a second violin section, we really shouldn't be having to fake a second violin section.
Often, the only sounds I use that are not VSL is drums. I know we have Jazz Drums, but it would be perfect if VSL had a full drum kit - though drums probably would need an exclusive player (like Vienna Imperial has for the piano).
oh, and something the OP suggested - I would +1 the accordion - that's quite Austrian isn't it?