Beautiful mantel! You could try a horizontal framed mirror, just overlapping the shades and slightly higher than the window rods. Just make sure it reflects something attractive. You could also try a collection of old books placed in the middle with covers in your color scheme. Arrange them not too structured to give you a natural look. You can always place a unique vase on the books. Happy accessorizing!

IMO, There's too much rigid symmetry here -
Try a single lamp on one end, a chunky vase on the other end, a trio of other items off-center - and rather than hanging your painting, lean it against the wall.

You need the ROSE WINDOW MIRROR from the POTTERY BARN

Thank you everyone for all of your helpful input!