If you could go back, and perform or leave the sheet music from some of the Modern era (50's-Today) music to inspire or give thanks to the Masters.  What genre would it be, Rock, Country, Soundtrack score?  Keep in mind that no electrical equipment back then and no real lyrics to speak of.  I'm looking for examples not just song titles; they are out there.  Personally, I would pick Rock and go with an older Metallica cover.  And, go:

https://www.youtube.com/watch?v=x97f-_y93a0&list=PLIiarIds5glX0GEPzxHatoXmcvb1-GjIF

OK, I'll bite.

Remebering that the classical composers were products of their time, what new melodies or rythyms would you expect for them to incorporate into their works?

Even as late as 1913, Stravinsky had some slight difficulty in introducing the new, with the non-acceptance by audiences of his Rites of Spring.