Batter made with pumpkin or squash, both traditional fall harvests, produces a delicious, moist cake. Applesauce or chopped fall apples add moistness and can also substitute for butter. Stir mulling spices (cinnamon, orange peel, and ground cloves) into apple batter to create unique "spiced cider" autumn theme wedding cakes. Or add a caramel-ganache filling for "caramel-apple" wedding cake.
Fall flowers are popular decorations for autumn bridal cakes. Asters and mums are the easiest blooms to find, and both come in shades that would blend well with a fall color scheme. Autumn leaves are another possibility. You'd probably want to use a fake version, as leaves can be brittle and bits could crumble onto the cake. These are available in many varieties and colors, printed with food coloring on edible "wafer paper." Check online for some good sources.
Acorns and miniature pumpkins, gourds, and squashes can be modeled from marzipan or sugar paste (a fondant-like mixture) and displayed against a smooth, white fondant background for an elegant effect on a fall wedding cake. You could cover the top tier of the cake with wafer-paper leaves, then top this with a sugar-paste cornucopia spilling over with harvest bounty.
Wine lovers might choose a wine-cake recipe, then decorate the top with bunches of modeled red or purple grapes and two wine glasses. If your budget will allow, you could have your fall wedding cake designed to mimic a wicker basket "filled" with modeled sugar-paste fall flowers and leaves.
If baseball is your thing, why not have your wedding cake reflect the start of the World Series? For an informal reception, some people prefer a sheet cake to the more traditional tiered kind. Imagine such a cake iced in a baseball-diamond design, complete with bases and pitching mound. A cake maker skilled in modeling could create edible (or saveable) bride and groom toppers portrayed in the roles of pitcher and batter. Who would be pitching, the bride or the groom? This autumn theme wedding cake would be quite a conversation piece!
You'll want to extend your fall look past the cake itself to whatever you decide to put under or around it. Consider a gold (or copper if you can find one) paper doily rather than white or silver. Surround your autumn bridal cake platter or tray with autumn-themed items such as small pumpkins, fancy gourds, or real oak leaves and acorns. Small white pumpkins with autumn-hued ribbons tied around their stems would be a nice fall complement to a white cake. Or mirror a cake-top cornucopia with a similar wicker version in the background.
If you can find a cake server and knife set designed for a fall theme, go for it. Otherwise, you can make your own to match your cake. Check craft stores for artificial leaves, bunches of grapes, flowers, etc., and tie these onto the server and knife with the appropriate color ribbon or raffia.
At a loss for ideas? Look for back issues of craft and homemaking magazines for the months of September through November. You won't find ideas specific to weddings, but you will find lots of autumn-related ideas that can be adapted to create a wonderful and unique autumn bridal cakes.