Sounds like an interesting idea! I'm sure there are already quite a few useful snippets of code out there, maybe a fun community project could be to collect and index those somewhere. I have some in-memory masked sprite drawing routines in my entry from last year, to contribute.
Concerning graphics primitives: most people try to target the older systems, or something "spiritually compatible" with those clock speeds. As such, when you need a circle, you pre-draw a circle and store that as sprites, because computationally drawing a circle is just way too slow :D But nevertheless, having line/circle/square drawing routines is probably useful for someone targeting XO-CHIP.