I have a couple of ideas bouncing around, but they might be too ambitious (plus harder that I use a less common language, often via bindings).
The first is a lot of ideas for a mesh editor, but not sure where to start. Technically, I already started a while ago with a text-based* polygon format/loader for Raylib so if I had some (high-level) way to export that polygon to a 3D file (single/double-sided faces or extruded) that'd be the first step. However I never really used/continued what I made (2D) because of how windings (fan vs strip) are, not good** as you'd need to keep track of it during usage. Perhaps something else might be better, like p5js?
The other is a frameless+floating X11 WM, and I'm similarly unsure of what I should use for drawing a title/titlebar and window buttons (and inset controls if possible). .xpm for bar/buttons is a maybe.
* sparse (a rectangle is 4 numbers on an otherwise empty grid), not like traditional (filled) ASCII art
** maybe expecting winding from an unordered list of points is too much, but not sure there's an easy way to cover windings for most situations. Color index (8-way) used as face hints, or similar (4-way?) for inside direction? Multiple origins for raycasts?