How about a wide variety of herb plants?

I second the herbs. In the fall a lot of people are canning their harvests, so dill would go over well!

Some pretty plants that bloom during fall are begonias, fuchsias which bloom their hearts out til frost, brugmansia (angel's trumpet), flowering maples (abutilon), japanese maples always go well, succulents such as echeveria and sedum, succulent wreaths and dishes, mums, grasses, mexican bush sage!!!, elephant ears, princess flower, some azaleas bloom in spring and fall, streptocarpus (indoor plant looks like orchid), bonsai, naked ladies (amaryllis belladonna), euphorbia (spurge)....

We sell mums and asters, up there maybe you could sell pansies for fall planting but here it gets cold too fast for there to be too much of a market for them in the fall. The black and orange ones sell ok here though.
We also start offering smaller sized baskets of houseplants like wandering jew(sorry if politically incorrect, I don't know another common name for it), and spider plants and pothos.
That is if you want to go into potted plants.

Thanks for all of your suggestions. I should have mentioned I do sell a good variety of echeverias, aeoniums and the like. I added these to increase my season, and it has helped. I am looking to really increase my offerings of hardy sempervivums. I'm looking for some that are different than what can normally be found. Any suggestions there would be most helpful. I believe I'll look into pansies, though I've had trouble getting these going in the past. I really appreciate everyone's help.

Teresa