I think you've got some great ideas to start with! I think the thrift store prom gown/bridesmaid dress is a good foundation. Especially if you can find green AND a full skirt and puffy sleeves. I would do a tattered hem line and then I would put slashes in the sleeves with a coordinating sheer fabric underneath.
Also, I just read an article on using fabric flower petals. YOu could use the petals like bead and "string" them on a length of satin cord like a long garland that could become a long necklace type thing. Also, glitter on all your cut edges on the dress and the under fabrics. Then you need to think about wings! Check out this tutorial from Simplicity: http://www.simplicity.com/index.cfm?page=section/classroom/sewprojects/wings/wings.cfm
Good luck- I can't wait to see what you come up with!