I’d be tempted to do feathers in the black. But I love feathers. You could do them from bottom to top or start with a figure eight in the center of each area and work out in each direction. Alternately, I’d think about a Celtic knot border pattern in each black area. The stencil company will have some that you can look at for inspiration or purchase from them.
Pick a couple of flowery looking motifs for the squares. Or suns.
I think I’d follow some of the swirls in the black/gray part of the border and a wavy line down the center of the wavy line of flowers in the rest of the border.