The Bitonal Piano Sonata by Malcolm Hill is a 20th-century exploration of polytonality, featuring shifting tonal centers between the two hands. It is a challenging structural work that demands high levels of mental and technical focus to keep the distinct harmonic layers separate.
The main challenge is maintaining independent tonal centers in each hand while executing complex rhythmic overlaps.
Comparable difficulty and character — good candidates for what to learn next.
Bitonal Piano Sonata is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 6 — meaning you can already play level 5–6 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Bitonal Piano Sonata lasts about 10 minutes.
Bitonal Piano Sonata is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Chromatic Fantasia and Fugue in D minor by Johann Sebastian Bach, Partita No. 2 in C minor by Johann Sebastian Bach, Basso Ostinato, VL 267 by Mikalojus Konstantinas Čiurlionis — see the full similar-pieces list on this page.