Mozart would have probably loved hip hop and metal!
According to Buck Rogers music of the future will sound like sleazy disco. good enough for me.
As a mildly interesting aside, electronic dance music has been standing still since the Prodigy's "Fat of the Land" (listen to that and it still sounds more modern than anything around today).
Stravinsky is probably the most 'futuristic' of composers, with his stylings being used by John Williams, Frank Zappa, and all those that follow in Frank's footsteps.
But, yeah, how can we predict music of the 6th millenium?
As a final note, one of the limits to musicians and composers before electronic sound production was the kind of noise an instrument would make, not just the notes it can produce, so alien instruments producing noises never heard before would be adopted enthusiastically.
Check out Bear McCreary's BSG soundtracks for a contemporary example. Rock guitar with kodo drums and bag pipes, middle eastern horns, and all sorts of other ethnic sounds all melded together.