What is the most difficult piece on the piano?

I need a bit of help... I'm 26 years old, and have been studying the piano now for 18 years, and have reached a dead end. I have learned and mastered most of the Beethoven Sonatas, La Campanella, all of the Rachmaninoff concerti and Hungarian Rhapsodies, and MUCH more. I would like to know if there is anything that you could reccomend that I study for a true challenge! My coach is unfortunately rather stumped on the matter. I know almost everything from Chopin to Liszt to Mozart to Haydn etc. I feel as if there's nothing left! I have learned and mastered almost all of what many pianists consider to be the most difficult pieces, and am looking a bright, new challenge! Any help at all would be greatly appreciated