Scheme a theme
If you’re not throwing your party on October 31st proper, consider a costume theme. It’s a great way to unify the party, and most people can put together a fun, low-key costume that fits, whether it’s a movie everyone loves or something more interpretative, like puns, abstract ideas, or unrelated combinations. It takes some of the pressure off to have a great costume and just have a fun, silly one.
Fair warning: this one depends on the quality of your friends. If you think they’re going to half-ass it, you could be left high and dry... and not just because your abstract costume is “High & Dry.”