The Graceful Envelope contest has posted the theme for 2017. Pushing the Envelope. That is a recycled theme from the very early days when the contest was sponsored by the Smithsonian Postal Museum. I entered more than one envelope that year - one made of sandpaper and one covered with aluminum foil. Those italics are horrible. I really liked the foil one and might use that idea again. Both were done in haste.
The deadline to mail isn't until the end of March, so you have plenty of time to brainstorm.