If your looking for something more mainstream and popular, black and white, hawiian, winter wonderland, moon and stars, valentines day, nature (outside wedding), and ethnic, and country weddings are all very popular. Some other different ideas that ive seen over the years decorating wedding cakes, halloween, day of the dead, star wars, princess and prince, fairy tales, alice in wonderland, wiccan, 60s, rock and roll, pirates, candy, rocky horror picture show....
Periodical era as a wedding theme would be awesome eg: Cleopatra, or the goddess and gods of Romanians would be intriguing. Or, re-acting Barbie and Ken, with a horse and carriage would be fun....I think it would be boring if you have a pink or blue or purple theme, almost everyone uses that.
When I worked as a chef at a ballroom, I catered a lot of weddings...some themes I encountered were: medieval (pretty popular), pirates, ethnic (Indian, Japanese, etc), The 50's, Western, and of course, just general decorating themes like butterflies and frogs (no lie, lol)
gothic wedding? i saw that last night on a halloween show, it has the minister as the grim reaper and the groom is a furry guy and the minister says "you may now pet your husband." that was the creepist thing ever XD
-Under the Sea Wedding