To my knowledge, the only way to get seasonal clothing items is by participacting in the Christmas Event, or an admin gifts it to you.
There are a lots of clothing items that we don't get in Christmas Event either.
What if there are several more mission like the Green Goo/ Alien Goo one, by which players can claim Seasonal Clothing Item Sets, like Halloween set that includes Cauldron and Witch Hat. In that way it would be more interactive and fun IMO.
If anyone has any other ideas of achieving seasonal clothing items then please mention below.
Thanks