As a landscaper I am a bust lol. But we do have a member who is real good at that sort of thing. His screen name starts with a "H" I think and he is frequently down in the plant identification area. You could probably find one of his posts and e-mail him.
Also, pictures of the area might help.
I would probably go for a roman ruin type look where the slab is. Using ferns and shallow rooted ground cover specimens just dump loads of dirt on it and let the plants do the rest. With a couple of stratigically placed pots where deepers dirt is necessary.
Where you've been dumping your clippings I'd probably build my compost pile. And maybe enclose it with boxwoods if it looks bad to you. I have mine over in a corner of my garden. That way you could use the compost for your slab area. I love composting.
Boxwoods are great for making "outdoor rooms" Was that what you were considering?
I love specimen gardens so any excuse I have to add a new plant or variety of plant and I go for it lol. So mine looks like a hodge podge.
Here are some possible plants for your area.
Acer shirasawanum 'Autumn Moon' Maple, Fullmoon; this is a short maple that is slow growing
. If you were willing to prune it so that it grew right up against your retaining wall or in a two demensional form this would be great possible over by the childens area? This gives 3 seasons of interest but of course would be bare during the winter.
Monkshood and lentel roses
planted at the base of the tree would give some additional colors and the lentel roses
bloom in winter to give some color and make a focal area during the winter months.
Actaea r. 'James Compton' Bugbane
If you can dump some soil between the concrete pieces could you put some ferns here and there amongst them? Make them look like an old roman ruin or something like that? that could be fun.
Maidenhair Fern, Golden
Himalayan Maidenhair Fern
Athyrium augustum forma rubellum 'Lady in Red'
Athyrium nipponicum 'Pictum-Applecourt' Applecourt Crested Japanese Painted Fern
Athyrium x 'Branford Beauty' Lady Fern, Branford Beauty
Athyrium filix-femina 'Vernoniae Cristatum' Lady Fern, Miss Vernon's Crested
Adlumia fungosa, Mountain Fringe planted at the base of a witch hazel tree treated like I mentioned for the maple would be a nice look.
If you do need a vine I would definitely look into this one. Akebia quinata 'Amethyst Glow' Chocolate Vine
Amelanchier alnifolia Serviceberry this is one bush that you might be able to use in place of the boxwood. You would have to prune it once it started getting larger.
Aronia melanocarpa 'Viking' Black Chokeberry
Wormwood becareful if you go with this one. It will damage other plants that are close to it. I have some in my garden and the daffodils that it is right next to don't bloom. I have to get in there and move them. But six inches away and they act like there is no problem. Mine is in it's second year and is easily 36" tall and has keep its leaves all year.
Arum italicum, Arum this will add winter interest to that area.
Astilbe a. 'Kvele' False Spirea color all year
Here are some boxwood that can handle part shade
Buxus x 'Green Velvet' Boxwood
Buxus sempervirens 'Suffruticosa' Boxwood
Buxus sempervirens 'Variegata' Boxwood
I hope you find something you like or that gives you an idea that you like.