My 2yo chose his costume this year -- he wants to be a robot. My 6yo wants to be a butterfly, and my 10yo was thinking about being a Chinese dragon with a friend, but has chosen to be Gonzo the Great (from The Muppets) instead... neither of them wanted to be the tail end of the dragon. Making a robot will be rather easy, and we ordered butterfly wings a few days ago, and the rest of that costume won't be hard. Gonzo is not going to be easy though.
I needed to buy a superman costume once, mostly because I decided that the it would take far more in materials and time to make it than to buy it, but mostly my kids have not chosen character costumes. I think it helps that they know I sew very well, and can probably make them anything they want.
Which reminds me, I need to get started on these.