Some properties (e.g. P22, P171 etc.) are easy to have value cycles (A=>B=>C=>...=>A). Maybe this can be prevented.
This has been identified in the 2020 report on Property constraints (T244043) as the most demanded constraint type by active editors by a wide margin. Among other Properties, this constraint type would apply to the Property "subclass of" (P279) and therefore help to improve the Wikidata class hierarchy.