But, If I wanted to go out with a bang, I would do an avant-garde piece that called for blowing up the piano (I wouldn't play it on a real piano though, just a keyboard that has Fur Elise as the demo piece)
As for me, maybe something really long, so that it gives me more time before I have to keep the piano away for good. How about the Sorabji OC?
If you knew that you could only play one piece on piano before closing the lid forever and never playing again, what would that piece be?