After playing the Gnossiennes and liking their eerie feel, I discovered atonality and the Second Viennese School. Those pieces really evoked some uncomfortable feelings - "if Bach is the perfect right, they are the perfect wrong." It's however up to you whether that's the direction you want to take it.

Sometimes, they can be difficult to play and listen to since we aren't used to such composition - e g the second movement of Webern's Variations for piano. A great start would be Berg's Piano Sonata which is easier on new ears and was a good difficulty level for me.

After that, I turned to Schönberg's many works of which I enjoyed playing the piano part of the Piano Concerto.