It would be such a joy to travel to 17th century Leipzig and tell the harried J. S. Bach that today he is arguably considered the greatest composer of all time and that his music is probably heard by someone every second of every day (just like I'll be performing his Mass in B Minor this weekend).
Or to tell William Shakespear that he is the greatest playwrite who ever lived to people in the 21st century and that even people who have never seen one of his plays may have seen "West Side Story" or "Ran" or even "Shakespear in Love" or used a phrase like "in my mind's eye" or "a rose by any other name ...".
Elvis probably knew how important he was. But it's not too late to let Chuck Berry and Fats Domino know.
It's too late to tell John Lennon. I'm sure Paul McCartney knows. But tell him anyway ;)