My advice would be to have a mixture of evergreen(look nice in the winter) and flowering plants that come back each year like alliums, phlox, lupins, peony's. Hebes are green all year and produce small flowers in the summer, have some spring bulbs like daffodils, snow drops, crocus, hyacinths and tulips which give your garden spring colour! Rhoderendrons are beautiful in early summer lavender always looks lovely and smells lovely in the summer! Here's some photos of my similar strip in my garden! The tall lollipop trees are flamingo willows they produce beautiful pink leaves/flowers. Good to have different heights of things too!