For a Christmas potluck, I would recommend bringing a festive and delicious dessert such as a fruitcake, a Yule log, or a gingerbread house. You could also bring a savory dish such as a turkey with stuffing or a ham with potatoes. Additionally, it's always nice to bring some festive beverages such as hot cocoa or mulled wine. If you want to keep the Christmas theme going, you could also bring some Christmas-themed decorations or tableware.