Franz Liszt's tribute to J.S. Bach uses the B-A-C-H motif to build a monumental work that progresses from somber, dark textures to virtuosic brilliance. It is considered a staple of the Romantic repertoire for its fusion of historicist polyphony and pianistic grandeur.
At Henle 9 this is among the most demanding works in the piano literature. The main demands: independent voices that must each sing on their own line; big chords that need weight without harshness; and fast passage work that stays light and even at tempo.
Beyond that, expect sharp dynamic and character contrasts held together in one arc, and chord voicing — bringing out one note inside a handful of others.
At roughly 14 minutes, stamina and memory become part of the challenge — plan practice in sections.
The main challenge is balancing the dense, organ-like sonorities with the extreme technical agility required in the later sections.
Comparable difficulty and character — good candidates for what to learn next.
A level or two down with the same spirit — ideal if this piece is still out of reach.
Lesser-known works by the same hand, at a comparable level.
Fantasia and Fugue on the Theme BACH (S.529) is rated Henle level 9 of 9 — virtuoso repertoire, roughly concert / virtuoso level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 9 — meaning you can already play level 8–9 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 Fantasia and Fugue on the Theme BACH (S.529) lasts about 14 minutes.
Fantasia and Fugue on the Theme BACH (S.529) 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 Mephisto Waltz No. 1 by Franz Liszt, Ballade No.2 by Franz Liszt, Piano Sonata in B minor by Franz Liszt — see the full similar-pieces list on this page.