I'm thinking more about the new garden planned in the front, where full sun makes many things possible. Scale and sustainability are important, leading me away from a crab apple (too big) and towards some shrubs and even an evergreen for four-season interest. Although this decision is based on looking at the lawn from the front of the house. When you look from across the street, it seems that the space is big enough for a small crab apple to work. (Of course, you also notice the shutter in need of repair - just squint and it will be less noticeable.)
But I'm also dreaming of planting lots of colorful perennials that won't thrive elsewhere in the garden. Inspiration comes from every mixed perennial border you've ever seen, and more recently from this display at the National Arboretum last June:
Echinacea - Just bought three 'Tomato Soup' - bright enough for you?? (No, this is not my photo, but when I bought them they were indeed this color.)
Really tall things, like eupatoriums, or perhaps vines on a trellis or stakes
Lilies - Adrian Higgins' recent column provides lots of inspiration
Rudbeckia - maybe this one touted by WFF
The first step is to lay out the bed and start mulching this fall. Then annuals next summer and planting in the fall of 2011.