As of right now I can figure out three alternatives to solving this problem.
Option 1. If you have some left over material from the original garment cut out a larger side pannel. it' isn't idea as you'll have to pretty much cut apart your whole dress and start over.
Option 2. Cut open the front of the bodice and add a contrasting fabric or similiar fabric to it.
3. Cut open the front of the dress and make it an open robe.
