A Fatsia in one, and a taller grass in the other....something to give a bit of contrast to the bright bedding plants

Almost anything! Are you looking for something that will look good over the winter, or some spring colour or what? (They are very nice pots!)

Hibiscus bush. Check my photo stack, I have a picture of one in a concrete urn about the same size as your pots. Actually there are two urns on either side of a old iron garden gate with hibiscus bushes in them.

Trouble is with Hibiscus is that they are the last plant to get their leaves and the first to lose them. The flowers are superb but just not enough interest for the other 9 months.

I buy mine fully leafed and flowered in May and they keep going strong till the cold weather stops them.

You've got so much fabulous summer colour, you don't need more, but you might enjoy some interesting shapes in the winter, so how about Pinus mugo? And if you don't mind pulling all the suckers off, a contorted hazel, in either green or purple.

If you like them how about a pair of clipped box spirals?
You could have some crocus round them for Spring.

Thanks everyone, loosestrife tried to find your hibiscus bush, could not find, idea sounds good, also winter colour sounds great.