Any of the Preludes and Fugues would be good. I do not know which ones you played, so it is up to you. They are all short but difficult pieces. They are wonderful for the budding young artist.
Chopin has 24 wonderfully written etudes. When played as a whole set, it is considered one of the hardest pieces in the standard repertoire. Liszt wrote many Concert Etudes in addition to the 12 Transcendental Etudes and the 6 Paganini Etudes. They are excellent pieces for the concert pianist. Czerny wrote two main books of exercises: The Op. 299 and Op. 740. These etudes are never to be played in concert or audition, but are wonderful for developing the strength and stamina of one's fingers, and is the gateway to building great technique in young pianists.
As for classical sonatas, Haydn, Mozart, and Beethoven wrote many great sonatas. Any one of them would be wonderful to learn and study. Again, I do not know which ones you played, and in the end, I recommend you try to learn it all!
For Romantic pieces, I suggest checking into Liszt Hungarian Rhapsodies, his three books of Annees de Pelerinage, or the Chopin Scherzos and Ballades. Liszt also wrote some Mephisto Waltzes, and check out his Sonata in B minor also. Chopin wrote Sonatas too; and while his 1st sonata is ignored, the second and third are widely studied and performed. Schubert wrote many Impromptus as well as Piano Sonatas. Rachmaninoff has some good preludes and Sonatas.
Study Prokofiev too! You will have to play him sometime! Why not now?
Topic: Repertoire suggestions