This may sound crazy but you COULD accentuate the bow by curving that outer border. I wish I could draw what I'm thinking of: it would look like a "square" with the sides being slightly concave rather than straight.
Having the dark outer border next to the light border also seems to accentuate that lower left corner. I had a similar situation once and solved the situation "visually" by couching a couple of rows of a pretty cording around the piece -- it covered the "bowed" section and also added some texture to the hanging.
That said....it's lovely just the way it is.