Here’s why I prefer making “useless” stuff: https://austinhenley.com/blog/makinguselessstuff.html
And my list of similar projects: https://austinhenley.com/blog/challengingprojects.html
I want to do a database and then something involving genetic algorithms next. If I have infinite time, another video game emulator too.
I had wanted to implement a spreadsheet, although I intended a "zoned spreadsheet", which has a zone grid (separate from the data grid), and each zone row has a number of data rows and each zone column has a number of data columns, and some properties are then properties of zone cells and others are properties of data cells. I think this would be better than most existing spreadsheet programs.
I did not know of some of the things that are mentioned there.