Need a zone or location.

You've already got a background. I'd suggest a relatively low (6" - 12") groundcover. It could be punctuated by some slightly larger colorful or blooming plants -- perennials or shrubs -- at the base of the foreground trees.

If it was me, I'd do a little more work on the wall, trying to smooth out the top of it a bit. Then, the ends, especially the right end does not look good ending like that. These ends should turn back into the slope.