Okay, here's another idea for those who like Winter Bash.
For those who participated in Winter Bash (got a hat) give a one-time badge.
You could give a Silver badge to anyone who participated, or
- Bronze - Up to 5 hats
- Silver - Up to 15 hats
- Gold - More than 15 hats
There is already a precedent of the
Beta Badge, for those who participated in the private Beta of a site (I got into SO in the public Beta and missed out on that one).