I put Heucheras in big pots last year they looked great all winter and have flowered all summer. A conifer, or a skimmia japonica or a Pieris would all look good too in large pots and give winter interest.

I would add some spring bulbs as well. Daffodils and other Narcissus do well. I found Aliums, Crocus and bulbs like Grape Hyacinth do not do well in pots due to the looser soil than one would get in the ground.

Evergreen grasses, for movement in the breeze, and winter pansies?

layers of spring bulbs for repeat flowering and last year my violas did better than the pansies