I'm going to be a mermaid, specifically modeled off Ariel's color scheme. I have a floorlength empire waisted sea-foam green bridesmaid dress and I bought a lavendar strapless bikini top.
My plan is to fold the bodice of the dress into the skirt to hide the embellishments on top, leaving a high waisted flowing skirt. Then (this is the part I'm still figuring out) I'll somehow tighten up the underlayers of the skirt to fit more like a tail, and wrap the gauzy outer layer around it to kind of smooth out and disguise the modifications.
My hope is that I can do all that without permanently changing the dress, because it's also exactly the right color to be the statue of liberty. I would have done that this year, but I was too lazy to craft a crown, torch, and tablet out of paper mache.