This work features virtuosic treatment of a popular 18th-century tune by the famous pedagogue and composer Muzio Clementi. The variations showcase idiomatic keyboard textures ranging from running passage work to broken chord patterns.
Achieving evenness in the sixteenth-note variations is the core technical requirement.
Comparable difficulty and character — good candidates for what to learn next.
21 Variations on 'The Black Joke', WO 2 is rated Henle level 6 of 9 — advanced repertoire, roughly ABRSM grades 7–8. In the RCM system it corresponds to about level 7.
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 21 Variations on 'The Black Joke', WO 2 lasts about 10 minutes.
21 Variations on 'The Black Joke', WO 2 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 21 Variations on 'The Black Joke' by Muzio Clementi, Fantaisie-Impromptu in C-sharp minor by Frédéric Chopin, Rondo capriccioso in E major by Felix Mendelssohn — see the full similar-pieces list on this page.