Hobby project I started, never finished, but I would like one day to pick up again:
* A statistics and probability manual that uses #DnD rules as both motivations and examples.
*A simple managerial videogame where you are a newly appointed academic and need to run your group, juggle teaching, admin etc
* A #GURPS-inspired ttrpg set in a world where magic is so common that everybody can use it.
* An app that allows you to make 3D diagrams for experimental setups just by drag and drop (without having to learn all the intricacies if Blender&co).
* An app that shows you the probability distribution to get a given result when rolling dice (e.g. roll 4d12 and keep the lowest 2).