These terminals properly handle single + double width fonts. But the same thing could work with single + half width fonts, so i,j,l,:,!,.,' and space would fit in half the width. Does anyone know of such a font? I searched but couldn't find any bi-spaced font with full/half width cells.

It would not only fit more text on a line, while (probably) also being nearly as pleasing as a proportional font, but it would also still perfectly align indents. I think it could be a great markdown font.