Well, that may depend on they type of soil that you have on that hillside. Is it clay and hard or sandy and soft, or ?
Also, do you want something tall or short? large or small?
Some Ideas I could suggest would be, for moist soil, cimicifuga, monkshood (acotinia?), larger ferns, ligularia, giant hostas, solomon's seal, goats beard.. tall or medium size (misty lace is a newer nice medium one), jacob's ladder, lady's mantle, old fashioned bleeding hearts and if there are areas where the sun pokes through, you could try turks cap lilies or maybe tiger lilies.

I planted muscari bulbs in my vinca bed and the result was a sheet of cobalt blue when both bloomed at the same time.