I love the idea of four seasons of interest and my suggestions are: Japanese maple, maybe a grafted dwarf with winter snowdrop bulbs planted at the base, or a dwarf Alberta Spruce with pansies around the base in spring and summer bacopa, or the orange rocket barberry with yellow and orange summer annuals and spring dwarf daffodils. You can also add heucheras, hebes, heathers, and hellebores to the container especially if there is extra room while the tree or shrub is still young. Make sure you use potting soil and a slow release plant food like Osmocote. It will be lovely. Keep growing, Marianne Binetti