I'm not entirely sure without re - reading it specifically, but there may be some minor characters introduced to add more flavor to the story, such as local villagers or new adventurers in the area.
In volume 5, one of the main plot points is the group's encounter with new challenges in the fantasy world. They might face stronger enemies or difficult quests.
The Konosuba light novel is not a fanfic. It has its own unique plot, characters, and world-building developed by the author's imagination and creativity.