A printer or printer firmware.
There was a discussion about this elsewhere on lemmy, of course this would be difficult and expensive but it would be very cool
Some of the OLD HP Thinkjet printers were pretty rudimentary; the original Thinkjet cartridges are still widely manufactured for certain industrial applications. Tell me we couldn't reprap that shit.
A reprap style project could probably make a passable document printer- but what's the appeal? People only work on those projects to make new or previously unobtainable machines available.