I like the idea of a map of North America with a pearl for very chapter. We can't forget about our outstanding Canadian sisters!
Maybe you can get pendants of all the chapters and put them around the room. Also, you can decorate in purple and gold with unicorns and irises. That can be visually stunning. You can also show the 85 year history of D Phi E from March 17, 1917 until now including our outstanding alumnae. I like the idea of a collage of postcards also.