Thank you, Lilli, Ian, and UndertheSun for your comments. I've since convinced myself to buy three spuria irises to satisfy my interest in this category of irises. The books say they don't bloom well in the Northeast, but the vendors tell me otherwise - probably because they're vendors!
I've refined the design to include these spurias, which apparently can become sizable clumps. The brown-looking thingies (winterberries, for winter interest) are gone for now, but I'm going to find a way to bring at least one or two back. I'm thinking of planting ferns and variegated Iris foetidissima in the back, which would give me red seed pods for the winter. Then Iris cristada in front of that, around the flowering center shrub - probably a hydrangea or hardy hibiscus, trained into tree form, for the architectural strength to balance out the evergreens and irises. Smaller perennials and annuals will fill out overly empty spots - especially before the clumps has established.
Now comes the hardest part - preventing myself from ordering more irises!
