Scented plants? Like thyme, lavender,oregano, mint (planted in its plastic pot do it doesnt spread) or Lilies, esp regale.
Flowering perennials: something to go up the wall in spring, like clematis montana rubens . Other clematis in summer.
Camellias, azaleas, rhodedendron, pieris, for spring depending on your soil.
A magnolia? Or a cherry?
If not ericaceous soil, then wallflowers, japonica, hellebores, erysimums and bulbs for spring?
Irises, Jasmine, a Tree peony, lilies for summer?.
Michelmas daisies, fuschia and sedums for autumn?
Calla lilies?
Japanese anemones?
Hollyhocks like they do in Giverney?
Agree with the topiary: cotoneaster is also good for topiary.
In short anything you like which means that something, preferably evergreen, flowers every season.