While the instruments and styles may evolve over time and styles, the basics of needing to fill the 4D mixing space do not.
In many ways, having to fill the mix properly with sitars and hurdy gurdys is not much different than having to do the same with guitars and drums or with trumpets and cellos or MiniMoogs and Arps. It really all comes down to the composition and arrangement first, and how they are or are not supported or improvised upon in the mix second. Each instrument has it's part to play, and the producer/engineer has to decide when and how to stage each one in a harmonious way.
Perhaps new instruments may mean that, at first, some canned mixing schemes (automatically put this here and that there) may not apply, but it doesn't - or at least shouldn't - take too long to listen and figure out both the instrument's role in the composition and how it fits (or doesn't) into both the arrangement and the spectrum.