Themes.

It would be cool for HOLIDAY EVENTS ONLY, for a limited time you had a chance to find the hidden easter egg theme. Once you unlock it you can use that theme for that specific season. Halloween theme would be fall, Christmas theme would be winter, Valentines/St. Patrick's/Easter could be spring, Then Independence Day for Summer. If you miss the window to unlock the theme for that season you can always try again next year.

Example: For Halloween, make a trick-or-treat scavenger hunt easter egg in discord itself. You have to navigate the menus and look for different pieces of candy hidden around by the devs themself. Once you find all 10 you unlock the Halloween theme. You can use the Halloween theme until the first day of Winter marked on the calendar. After that the theme goes hidden and you cannot use it until next Fall Season. 

I see this as a good idea because every season there is a new theme so people can't complain about not having it or missing it. If they do want to complain you can always remind them next season the same theme will be released. If you could seriously take this into consideration I'd be very thankful.