This year were considering Alice & the Mad Hatter or Cowboy & Indian...(not to original I guess). Last year one of my friends were Cloudy w/ a chance of rain. He had cotton glued to his shirt & she carried a mister to spray people. Was cute!
It depends on what type of party your going to. You could do Frankenstein & The Bride of Frankenstein, Hahn Solo & Leia, Bonnie & Clyde, Sonny & Cher, Elvis & Priscilla. I saw a great one online that was Dog the Bounty Hunter and Beth. Maybe Sperm and Egg?? if it's an adult party that is..
I normally get inspired by looking at online costume shops or just google couples adult costumes!
For couples I love the idea of going as an executed Queen such as Marie Antoinette or Mary Queen of Scots and having the guy be the Executioner. Or a couple could be Marie Antoinette and King Louis XVI with matching neck wounds since they were both executed.