You should definitely do the Ballade or the Mephisto. Probably not both as you really won't have time.Consider the order and do a trial run to see your time - my guess is your running a bit over the hour. Allow 2 minutes between pieces too, more time gets taken up there than you might think.If you do the Debussy, do the Ravel as well and save the Gershwin for your encore - it makes a nice contrast there.
Then I'd need an encore piece. Could you or anyone suggest a piece that would contrast with Beethoven, Chopin, Gershwin, Debussy?
Personally I won't end with Debussy..too slow, anti-climatic. I will end with Chopin Ballade or Winterwind, or the Preludes. Another comment: I do not like to break up set pieces...therefore I would wait to do the entire Valse Nobles when they are ready. However, it is your recital, you do what is most comfortable for you. Cheers.