Blumenfeld’s Op. 34 is a sophisticated fusion of the Ballade form with rigorous theme and variations. It utilizes a vast technical vocabulary reminiscent of his mentor, Anton Rubinstein.
The technical demand involves managing the transition between varied stylistic textures and the physical requirement of large chordal spans.
Comparable difficulty and character — good candidates for what to learn next.
Ballade in the Form of Variations (Op. 34) is rated Henle level 8 of 9 — concert repertoire, roughly diploma level. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 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 Ballade in the Form of Variations (Op. 34) lasts about 12 minutes.
Ballade in the Form of Variations (Op. 34) 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 Allegro de concert by Felix Blumenfeld, 'Sur mer' (Etude) by Felix Blumenfeld, Ballade No.4 by Frédéric Chopin — see the full similar-pieces list on this page.