You might look into a dwarf oakleaf variety of hydrangea like 'Pee Wee' or 'Ruby Slippers' to offer some summer/fall interest. The blooms start out white and age to a rosy pink and the leaves turn a pretty red in the fall. The peeling bark adds a bit of interest in the winter. Just a thought!
There are different varieties of asters, some a low growing and some are quite tall so you should have options. Lenten rose may offer some late winter/early spring interest to extend the season in the other direction.