Isn't that more like switching to a different character set,
with supporting fonts having to be already present? Or what are you referring to?
I was thinking along the lines of online (or inline) fonts, similar to web fonts in CSS, which would allow for arbitrary custom glyphs.
Sixel graphics on the vt220 (1983) worked by defining a custom font of 10x10 px characters (9x10 in high-resolution mode), which you could then switch to for any given character.
As far as I know, there is no terminal emulator that supports this flavor if sixel graphics.
Personally I’d rather see in-lined vector graphics become better supported. This was available on some of the Techtonix (iirc) hardware VTs and as far as I know, the only widespread terminal emulator to support that particular mode is xterm.