Special holiday events that would have skins for players, weapons & enemies. Kind of like what Killing Floor 2 does. For example a Christmas event (limited) where the characters get special skins and you can only get them only that time and limited double XP week/weekend. All the enemies would be dressed up based on what holiday event it is too (Halloween, Christmas, Valentines Day, Etc.) Maybe even throw im a special horde level only for the holiday events.