Parsing code.
PEG
Development workflow using Docker.
Dev container.

Programming languages
Unicode, i18n
Networking, HTTP routing, REST
Web frontend, ProtoBuf/gRPC
Formatter
Linter
Templating
Resource embedding
Cross compilation
All-in-one single binary

LPeg.
Minimal source set to build Lua for LPeg.
Unit test.
Coverage.
Static code analysis.
GitHub Actions.

Math in doc.
Math in Markdown.
LaTeX to HTML.
Math rendering.
Math font, and font rendering in general.

Simple web page design.
Frugal CSS.
Frugal HTML.
Accessible design.
Super minimal static site generator.

3D drawing app.
WebGL JS.
Draw objects in 3D.
Draw mechanical systems.
Draw things for cooking.
Draw music related stuff.
Draw kanji etc. in 3D.

Music notation app for hobby use.
Intuitive interface.
No need to scale for orchestra use.
Small footprint of app.
Web interface, maybe in JS.
Simple sound generation.
Super simple piano app.
Chord progression / chord sequencer.
Music font (SMuFL?).

Simple logging app.
Simple graphing.

Food and health website.
Cooking recipes website.

Kanji katakana hiragana learning web site or app.
Stroke animation.