You can write a sequel or story 'what happened before' your Christmas story. You can write a story about New year - celebration, resolutions, etc. You can join Storytelling Contest and continue story about Advent Calendar. You can write about Brain Tree during Christmas time. (Few days I'm daydreaming about Brain Tree's outfit for Christmas time.)
You can write anything that comes to your mind, even if that's a short paragraph that makes no sense. Who knows - maybe some day you'll figure out how to combine few delirious paragraphs into amazing story.
What things do you usually enjoy writing? Fiction or nonfiction? Long or short form? Would you want to write something similar to your Christmas story, or were you thinking of trying something different? Figuring the type of story you want to write could also help narrow what it would be about.