What: 3D meshes (edited in 2D), vertex color.

The simplest example would be to start with something like a triangle or hexagon grid, being able to paint each cell to create a design, then export painted cells as an optimized mesh (glb/obj). I know there are probably a few ways to do this manually, though likely not very smooth. Perhaps even involving SVG conversion (either way, likely involving Blender).

Why: Billboarded characters/projectiles, items, paintings or other environment details, floor/wall/ceiling tile art, UI icons. This would also give a more consistent/stylized look than manual meshes.

How: To me it seems like the newer node-graph applications (Graphite, PixiEditor, or even Blender's geometry nodes) could allow someone to create an editor to do stuff like this (perhaps as an extension).

Other related ideas:

• basic live-unwrapped 3D support (cuboids, revolve/lathe)
• quick patterns (tessellations, spiral vases, futbol)
• semi-freeform cell placement mode (think wooden pattern blocks)
• library tools, particularly indexed color palette+export all
• multi-layer for more flexibility, could be overlapping (obscured verts may not be exported) or not (flat+offset, or multiple extruded parts)

Somewhat similar thing, I did see that Joey Carlino made some character creators (Spore, Mii) in Blender.

I may eventually attempt something, currently stalled (system issues) and the last polygon thing I tried was a text format/loader (via Raylib bindings) that I only sort of completed (really only usable for a very simple project).