I've seen all kinds of costumes from Disney to non-Disney. You will not feel out of place in a non-Disney costume!
I've seen pirates, mideval princesses, a family of Elvises and poodleskirt girls, Peter Pan groups, Wizard of Oz unit, Flintstones, Strawberry Shortcake, Superman, Spiderman, tons of Disney princesses, Alice, hitchhiking ghosts, witches, dalmations, Kim Possibles, Harry Potter kids, Yugio, devils, I Dream of Jeannie, and more.
The best way to stand out in a Disney costume is to put together your own version rather than using store bought, or choose a less popular character.