How to add some interesting detail to this dress?

If you want to add something to the dress itself, not just an accessory then add some black crystals to the neckline of the dress for subtle sparkle. If it were one of my designs I would add some details to the neckline and the arm edges, perhaps satin cord outlines, crystals, or sprinkle black sequins on the bodice, something to break up the large expanse of plain black. That's a lot of plain matte jersey and it needs something to add life and interest. I would certainly add something to the neckline to draw the viewer's attention up the the wearer's face. If you want a belt, then a shiny patent belt with a large buckle worn high on the waist, emphasizing the narrowest part of the body -would be very trendy. Keep the belt the focal point and go small but sparkling on the other accessories.