The specific additional VI I feel need to be sampled and programmed consistently with the previous VI series are:
Dimension Stings III with ponticello sustain and tremolo, glissandi, spiccato, sampled trills like the Orchestral strings extending beyond half and whole tone intervals (performance trills are no substitute for sampled trills most of the time) and ideally a series of unusual string effects. Having FX on Dimension strings would be mind-blowing.
Historic winds - SACKBUTTS! Absolutely essential for so many historic era performances.
Historic Percussion - the only option now is to use modern percussion instruments - like modern mechanical timpani etc. - a very lame compromise for totally different sounding authentic historical drums and cymbals.
Historic Strings such as Viola de gamba, rebec, theorbo etc. etc. - tons of possibilities there.
It would be completely wrong to do these important additions with a system that did not fit into the previous vast body of samples, so it is essential to produce these first as additional VI.