Looks great! I would like to do something like that at our new place. I think I would add something that looks like a small grass or moss looking plant.

Depends on your climate, but mosses and rock flowers work well in that sort of spot. Nasturtiums will also cling just about anywhere that's sunny, and will give you a gorgeous sprawl of yellows, oranges and reds - just throw around a few handfuls of seeds once all risk of frost is over.

Go to your local nursery and ask about ground cover plants to hold the soil in place.

One thing you're going to want to be careful of is moss growing on the steps themselves as it can be a slipping hazard. Baking soda [1/2 cup to a gallon of water] is a natural moss killer.

the fastest and the easiest to propagate are succulents ... but get as many varieties as you can, and keep propagating an planting all over until filled up ... also good at keeping some weeds down.