|that will be sole focal point of 20x20 terrace with tan stone wall to its back. full/partial sun. zone 6.something inconspicuous but extraordinary.|
|You might consider something with a blueish tone to the foliage such as one of the creeping junipers such as J.procumbens nana. This is a particularly elegant low mounding form, and would do well in the hotter sunnier area, especially with that reflected heat from wall and patio. In the partially shaded area you might try creeping wooly thyme. This also has a blue tone but a different textural accent.
Another option might be Ceratostigma, a deciduous perennial that forms a carpet of foliage for most of the growing season. This plant turns red in the fall and also produces glowing blue blooms in late summer to early fall. It is tolerant of a variety of growing conditions and would tolerate sun and partial shade.
If the lighting is drastically different, I would suggest using two different plants because the growth pattern of a single plant will vary due to the lighting difference.
Midway, to bridge from the height of the tree to the low patio/groundcover you might try some shrubs massed to created a drift. You could consider deciduous shrubs that turn red in the fall such as Itea (this also has lovely white blooms) to enhance or accent the Katsuratree coloring, or perhaps a deep green, needled smaller evergreen such as the mugo pine for a contrasting texture.
Another good thing to consider would be smaller spring flowering bulbs such as crocus. This will add an extra seasonal point of interest.
Last, you might add a grouping of large boulders to both echo the other stonework and create some visual balance and interest. Another trick is to use one or two very large oversized containers filled with annuals for a seasonal change and add some visual punch to the scheme.
My favorite for an understated effect is a reflecting mirror of dark water. The water is dyed and it need only be a few inches deep with a recirculating burble of water to keep it aerated. You could also have a simple stream of water jets coming out of the stone wall. The pool can be of formal or informal shape and the surrounding edge can be planted very simply in English ivy.
Enjoy your project!