Are you wanting shrubs, perennials, annuals or a combination thereof? Knockouts are so durable they can be combined with a vast array of other plant material. :-)
Perennials I have planted near my 'Knockout' include daylilies, salvias, verbena, shasta daisy, anise hyssop (Agastache), Russian sage, hollyhocks and several types of ornamental grasses.
Annuals usually near my 'Knockout' include rocket larkspur, love-in-a-mist (Nigella damascena), cosmos, zinnias, marigolds, and moss rose.
Basically any full to part sun plant which doesn't require constant moisture could be a possible companion! :-)