I'm surprised nobody came up with this idea before.
Hints/ideas:
1. It's just for fun. Christmas are celebrated only couple days in a year, so if you plan to invest a lot of time, it's better to do something generic (see hint #2)
2. Extra points for assets that can be reused in non-christmas themed dungeons (e.g. pine trees that can double as christmas trees, but would be later usable for forest wallset)
3. Anything even loosely related will do (fireplace, a window that you could look through and see snow outside, gift-wrapped boxes etc.)
4. I for one would welcome an angry, christmas hat wearing ogre that will say "ho ho ho" before smashing my party to pieces.
5. Red/white overweight goromorg with a large bag full of toys... er, equipment I mean could be interesting.
I'm afraid I don't have anything to offer as a prize here. I can buy a gift code for Grimrock, but all of you already have one. So for now it's simply for fun.
Here's something to get you inspired:
