The common themes include different versions of Earth. In science fiction about the multiverse, we often see Earths with different technological levels, like one where the steam age never ended, or one where humans have already colonized the entire galaxy. Another theme is the reimagining of historical events. For instance, a universe where World War II had a different outcome. Also, there's the theme of unique species. There could be universes where entirely new and alien species exist, and they interact with humans from our universe in unexpected ways.