What would you plant here?

[imafan26]

It is not wide space and the path itself is very narrow, more of a service path than anything else. Some of the shrubs would need to be pruned since they will go wide. If you are in a southern location 4 hours of sun isn't all that bad.

A trellis would probably give you a lot of vertical height with minimal width. If you go that way. Only use annual vines and do not let them go to seed. Many vines are invasive and will require pruning to keep the runners and the children in check.

Instead how about Italian cypress. It is a narrow tree. It grows fast and tall up to 60 ft. It's roots are not considered to be invasive and I have seen old trees planted on curbs that had to be topped because of the height but still did not cause problems lifting the sidewalk or road. It is hardy down to 10 degrees , but you do need to treat them for spider mites which will cause the needles to brown and die and I doubt that it will have a problem with over watering on a slope.

Otherwise if you are into art. I would do a faux painting on the wall, if you don't have an HOA to deal with. Even mounting a couple of fake windows and window boxes will make the wall less blank.