Any wedding with an autumn theme has a lot to work with. It can be indoors or outdoors, thanks to the cooling weather. The rich, vibrant colors create a broad palette to choose from, anything from deep jewel tones to subdued earthy shades. There's also two holidays that can be focused on, Halloween and Thanksgiving, or a masquerade and harvest themes.
My own wedding was in the fall, and I chose a general autumn theme, using vibrantly colored leaves as my inspiration.
Autumn Leaves: Ceremony
Instead of dropping flower petals, my flower girls were given wicker baskets full of silk leaves. They threw the leaves over their shoulder so they fell to the carpet.
Autumn Leaves: Bridal Party
Since autumn is full of so many vibrant colors, my bridesmaids dressed in the colors of the leaves. My maid of honor wore gold, and my maids wore a deep red and a rich chocolate brown, and the groomsmen wore matching waistcoats. More silk leaves, twigs, and acorns were attached to hair clips and headbands for the bridesmaids and flower girls to wear in colors that coordinated with their dresses. As a special gift, I bought each bridesmaid a real leaf dipped in gold, and put them on chains to wear during the ceremony.
Autumn Leaves: Reception
My wedding was the weekend after Halloween, which meant I was able to get several good pumpkins for a dollar each at Wal-Mart. I bought them a day or so before Halloween and kept them in a cool place until the day before the wedding, and had a pumpkin carving party with the wedding party. We hollowed out the pumpkins, then drilled holes in the sides and inserted clear glass marbles. We put pumpkin lights inside and used them as walkway lights outside the reception building and as table centerpieces. I carved unique pumpkins with silhouettes of falling leaves for the bridal party table and the parents' tables. (You could do table numbers, different words for "Love", or your initials.) The reception tablecloths were red for the guests, brown for the buffet and desert tables, gold for the bridal party. Our cake had three tiers, was frosted in white and coated in coconut, and featured silk leaves cascading around the outside in a gentle spiral.
Harvest: Ceremony
Consider sheaves of wheat, bales of hay, or cornucopias of fruit and squash for decoration instead of flowers. You could have the ceremony in a rustic setting outdoors, like a field with blanket covered hay bales for seats, or an old barn.
Harvest: Bridal Party
Wear a garland of leaves and vines instead of a veil with your white dress to set the tone of a classical goddess of grain and harvest. Mimic the headdresses for your bridal party, and dress them in tones of yellow, gold, and brown
Harvest: Reception
Consider traditional Thanksgiving foods for your reception dinner. Use cornucopias or carved pumpkins as table centerpieces. Carry your color scheme throughout the room.
Masquerade: Ceremony
If your wedding is near Halloween, you may consider having an elegant and sophisticated costume party, or masquerade. This can be incredibly flexible as far as color scheme, so don't feel you should limit yourself to orange and black. The hardest part will be making sure your guests participate, if that is important to you. Consider ordering masques online and having them available for your guests as they come in, or if you are particularly crafty and have lots of time, make them yourself! Watch movies like Labyrinth, Ever After, and Phantom of the Opera for inspiration on traditional masques.
Masquerade: Bridal Party
The masques you order or make can also be worn by the bridal party, in colors matching their clothing. I would suggest replacing your veil with a masque, at the very least. Remember to think of your bridal party's clothing as a costume while still dressing in formal attire; for example, many weddings feature flower girls wearing their formal white dresses with gauzy fairy wings in bright colors. Add an elegant masque covering just the eyes, and this is a perfectly acceptable masquerade costume. A person could dress in a brilliant blue, wear a masque with peacock feathers, and place one or two in her hair, and be a peacock at a masquerade. Consider choosing a theme for your masquerade wedding before you select your masques.
Masquerade: Reception
Consider a mansion or museum for your reception to get a location with a large ballroom. If chandeliers are present, use them to create a romantic evening glow. If a chandelier is not available, line the walls with standing candelabras to get a similar effect.
These ideas should get you started on planning your own unique and romantic autumn themed wedding. Congratulations and best wishes to you on your upcoming nuptials!