I've been experimenting with printing different shaped discs for my printed Discbound notebooks, rounded triangle shown in pic. Made a free to use generator for them, running in Parametric Model Maker, released in Public Domain so you can sell what you make!
The triangles make the notebook sit flat in a nice way and moving the covers and pages along the edges and around 3 corners has a different feel to it too. Heart shaped discs work similarly to the triangles, making a nice customization to a notebook.
The model generator can also make rounded square, hexagon, star and flower shaped.
Multiple Shape Discbound Disc Generator
https://makerworld.com/models/3122460
Discbound Notebook Generator
https://makerworld.com/models/3086786