I wouldn't plant anything else there, they all will fill in nice, there really isn't any room left. If you want something that blooms, why not plant clematis by the boxwoods. Let it climb where it wants, it will bloom and look nice and it plays well with others. You can buy small vines, and they will grow in with your shrubs. That way, you won't have to remove any shrubs in the future because it's too crowded.

You could fill in with annuals, perennials and bulbs until the shrubs fill out. And beware the meatball when shaping the shrubs. You can keep them shapely without destroying their character.