I love the idea of the Rio Mardi Gras! It would tie in well with the release of Breaking Dawn too.
Also like the idea of a policeman's ball or a Cops and Doctors party.
How about a Coven's party to celebrate Breaking Dawn? You could be a Cullen, Quilayute, Volturi, Denali, Irish, Egyptian, Nomad or a human? Could be a good way of celebrating everything we love about the books?