To try to guess what that might be: Strings and Brass both have Dimension libraries, and Herb has previously said it wouldn't be necessary for the Woodwinds. I doubt Dimension Choirs would be technically possible, but I've been wrong before. About the only gap in the VSL arsenal across the orchestral spectrum is solo singers, though again that could be very difficult to achieve realistically.
Another direction might be period instruments, like the Ophecleide and the Serpent, or ethnic instruments like the sitar, the erhu and the ocarina.
Pyre