try an evergreen shrub there are some lovely hebes then there are Skimmia etc

Cornus alba variegata, for winter stems, with a dwaft clematis growing up it for mid-late summer colour.
Underplanted with ophiopogon and cyclamen for winter leaves.
When available, tuck in some crocus, miniature daffs or scilla.
And heucheras do well in pots as long as you treat for vine weevil.

As nice as they sound, the pot is only suitable for very dwarf stuff as it's only 9" across and 11" deep.

Personally, I'd sow some night scented stock into it for the summer and have some pansies in the winter

I have put Heucheras in my tubs for the last three years and am pleased with the all year round results, the leaves are very pretty and the flowers can be a delight. There are lots to choose from but I now avoid the pale yellow leaved ones as I find these do not do so well unless in real shade.