Okay so I'm inagining a white dress on her, and to keep it clost to the costume. I think you should keep something clost to the top, but instead of it stopping as a top make it into a full length dress. where the side panels come in at the shirt, turn that into a full length dress that cuts off the main paneling with all the buttons. For the sheer over coat make it into an actual coat with sleeves that goes down to the floor. The Jewlery she is wearing allready is fine.
IF you want to make it absolutley spectacular you can turn it into a two peice ball gown by keeping the top, but adding in a red (or if you want to go plaid you can) ball gown with a 3 to 6 hoop petticoat and keeping the side panels the same but letting the main panel drip fown the front to the floor, keeping up with the buttons the whole way. Abandoning the sheer overcoat for a whole overlay of the sheer fabric you have on the red skirt.
~~~~~Current Con plans~~~~~
Anime Expo 2013-Bathing suit Maylene or Ciel in wonderland Maylene