Well the Rach Prelude IMO is more difficult than some of his etudes, and if you can play the full Grieg Sonata then I'm sure you could tackel most of the Rach etudes. - And yes, they do count, cuz this music was written in the 20th century. You should look at his opus 33, out of that set I liked #2 C major, 4 D minor, 5 Eb minor, 6 Eb major, 8 C# minor.
Or look at more of his preludes, op 23 #2 Bb major, 7 C minor, 9 Eb minor if your into double notes, Op 32 #1 C major, 3 E major, 4 E minor, 6 F minor, 8 Aminor, 9 A major, 10 B minor......... some of those are really short so maybe pairing some up, but not sure if you'd want to do that.
For Ravel the sonatine would be an excellent choice, but it is around 11 minutes. Also, once again, with your current rep, I'm sure you could play this. His tombeau de Couperin is underplayed, look at the Prelude, Rigaudon and Toccata from this. You might find the toccata too difficult but it's still an awesome piece.
Out of his Miroirs look at Noctuelles or Vallee des Cloches.
Gershwin also has three preludes which are excellent and probably your level. They should probably be played as a set.
Look at some Albeniz or Nazareth if you want a completely different style of music, spanish rhythms, tango type music - some of which is very difficult.
For Debussy I barely know the preludes, but I do know that L'isle Joyeuse (not a prelude) would be an effective piece, also look at his suite for piano, Prelude and Toccata.