It's not going to look good unless you cover the ground solid with plants. Where the Nandinas are, add more (probably triple them as what's there will never grow together.) For the area beyond the Nandinas, you might consider a flowering groundcover like Daylilies @ either 18" or 2' on center x 3 rows ... either hybrids in mixed colors or a single everblooming such as Stella d'Oro. Leave some clearance around the fountain and plant something very low growing--like what you have edging the wall--around it.

I love pollarded crapes, but your's are not examples of good form. They look better with a fat, full trunk structure. Then, why pollard at 4' ht.? This creates a foliage mass that is obstructive to the view beyond and not easily walked under (since you have a path nearby.) It would be better to work toward cutting them at a 9' height, by adding a couple feet of height each year. If these were mine, I'd start them over by cutting at 6" height and keeping a bouquet full of trunks when they return.