Come on guys, throw in some creative thinking..!
You could also regard the topic like this: what song would you have liked to hear at another point in time if only there was a chance (even if the song hadn't actually been written by then).
Examples: "Polk Salad Annie" in 1963? "I Washed My Hands In Muddy Water" in 1976?
The choice would have to be original and funny/interesting - that means this would be a bad example: have him record the entire soundtrack to "Harum Scarum" in 1975..
You could also consider bringing up choices that might have been impossible for Elvis to do. Understand his artistic mind and find out what wouldn't or couldn't work (at some given point in time). Example: an operatic "Unchained Melody" in 1953..