I think I would use it for winter pansies.

I was going to suggest geraniums but you presumable want something with winter interest. Pansies are fine, I find, but if you forget to dead head them regularly they soon go over. Once the weather is bad I don't give mine a thought unfortunately.

All plants eventually outgrow their pots but you could try heathers (need an ericaceous soil) which are colourful or maybe several heucheras which are also colourful. Try mixing Lime Marmalade with Apricot.

Do you mean winter bedding or a permanent planting?
If you choose pansies its not too late to add some early tulips - perhaps the Kaufmannianas? Put below the pansies they will come up among them.

Any amount of spring bulbs would be happy - fill in with winter pansies, violas or cyclamen.
Hostas for summer/autumn foliage/colour

How about a japanese maple. they are slow growing, come in many varieties. You won't need to prune the roots for at least 2 years. Also many varieties give fantastic Autumn colours.
A great growing medium for this is non-clumping semi baked clay cat litter. not cat litter made from paper or wood and not clumping type.
No, I'm not joking. i use this for growing bonsia (including Japanese Maples). It allows air to the roots (essential for good root health and growth), it cannot be overwatered as excess water runs through the pot while the cat litter retains enough water for the plants upkeep.

GOOD THINGS:
Cat Litter can be re-used;
is totally inert;
is lighter than compost;
and cleaner;
relatively cheap;
cannot be overwatered;
easy to check if watering your plant is required... just stick a wooden BBQ skewer into the cat litter, leave it for 5 minutes, pull it out. If skewer is dry, water plant. If skewer is wet or damp, do not water;
can be used for any type of plant (plants, trees, shrubs, cacti, etc.);

BAD THINGS:
For best results, sift out dust using a fine mesh e.g. flour sieve;
you may need to feed more often;