Love that wall. I need a retaining wall, but not in the cards right now, sadly.

As to plants, I would check out natives for your area. For suggestions, would need to know how much sun/shade the area gets and what zone you are in, and possibly nearest city/large town. I would create a bed of perennials that would flower from spring until fall. Although not native, if it is a somewhat shad spot, Iwould mix in some hellebores for winter interest if your climate suits them.