Ok, so here's how it went down.
The Girl, out of nowhere last April says "Mom, for Halloween, I want to be Dorothy. Can you make me a Dorothy costume?" and I say "Of course I will," but I didn't expect the next request: "Mom, if I am Dorothy, then Daddy has to be the Tin Man and you can be Glinda!"
At first I said, aw gee, I think that might be a little too complicated for me to make just for Halloween, but the thing that convinced me was the hoops. Glinda's skirt has a very 18th century silhouette, flat front and back and wide sides. "I could make hoops," thought I, "that I could then use for all kinds of awesome 18th century things!"

So I decided to do it. I did talk her into letting her dad be the Scarecrow instead of the Tin Man, because I really doubted he would wear a Tin Man costume, and anyway I didn't want to make one.
So, I have the pink shimmery organza (probably not enough but we'll see). I need to figure out how to do the silver stars. I probably will not make them as dense as the original, but I need to do some. The crown I will probably buy. The wand I can make. She is also wearing quite a bit of bling, so it will be fun sourcing that. All in all, this will be the most involved Halloween costume I have ever done.
And it might end up coming along to Costume College with me, though that was not in my head at all when I decided to make it. I thought the hoops would probably come with me, but we'll see how this turns out.
The most suprising thing is that she has stuck with this idea for months, and still wants to do it! I was so sure she would forget or change her mind.