These days, it doesn't look as if there are many limits when it comes to what can be done with ugly sweaters. Some of them truly have to be seen to be believed. Even the most clashing color schemes and silly patterns possible are honestly fair game.
That said, you will definitely see certain themes reoccur. Red, white, and green are among the most popular colors given their association with the holiday season. (Other clashing splashes of color are often added these days however.) Patterns and designs that incorporate traditional Christmas iconography are also common. Examples include snowmen, holiday trees, gift boxes, ornaments, snowflakes, cups of hot chocolate, milk and cookies, and Santa Claus himself. The fun comes in how many of these elements might be incorporated all at the same time. There's also a sweeping trend of using this basic style template in tandem with pop culture figures. If something exists, at this point there's probably a Christmas sweater of it!