Iris, chrysanthemums, roses, waterlillies, the point is not to have too many and to have them in the correct season. I understanfd that the plants are often grown in pots so that the correct one can be moved into the garden for viewing at the correct season and when the plant is perfect. Do we have any Japanese contributors?
Definitely some kind of water feature. If a pond - make sure it has Koi fish and water lillies that produce fragrant lilly flowers in the summer. Or alternatively a simple stone or pebble fountain feature could be nice. There is even a bamboo one I've seen that periodically channels water into a tipping device that delivers the water to a second level - very soothing - very zen!
Acers are the best and most stunning of trees used widely in japanees gardens but seek advice as to which acers will be most suitable for a particular location as most do not like direct sun.
Bamboos and some sort of ornamental peach or cherry tree with good blossoms in the spring. Japanese Maples, Azeleas (sp?), Gravel. large rocks, definititely a water feature, wind chimes some japanese lanterns? OK OK! I know they are not all plants... ;(
This might sound a bit daft but here goes - moss. Japanese gardeners often seem to employ moss the same way Western gardeners use grass in lawns. A few, thick patches of moss can look quite luxuriant. I also remember seeing a gardening program where they suggested painting statues and rocks with a water/manure mixture to encourage the growth of moss and lichen on them.
Texture is definitely important. A nice grass that grows in hillocks is called Korea grass -- humps, hummocks, all very contained. Do consider raking a 'river' of sand around some largish rocks to create the illusion of water. Google for precise garden elements!