Step 2: Transform what you've got
My sister, who is a much better seamstress than I, and much more willing to spend an obscene amount of time making costumes, had made a beautiful white kitty cat suit for my niece. The next year my boy would not hear of being a cute white cat, but he did like the idea of being "the sugar monster." All it took was a blue cape trimmed with (empty) sugar packs, a few other sweet (literally) decorations, and my son became a very enthusiastic sugar monster. Nobody got it, of course, the idea was a bit too abstract, but he still got lots of candy.