I haven't read this volume specifically, but generally in the KonoSuba series, it often has hilarious and over - the - top adventures. Maybe this volume continues with the misadventures of Kazuma and his party in the fantasy world.
The main characters in the KonoSuba series, which likely are also the main characters in vol 16, are Kazuma, Aqua, Megumin, and Darkness. Kazuma is the cynical and somewhat perverted protagonist. Aqua is a useless goddess. Megumin loves her explosion magic, and Darkness is a masochistic crusader.
In Konosuba Vol 6, Kazuma is the leader - like figure of the group, trying to keep everyone in line. Aqua provides a lot of the comical relief with her lack of practical skills. Megumin's explosive magic is a key part of their combat strategy. And Darkness adds an interesting dynamic with her masochistic tendencies. Their different personalities clash and combine to drive the story forward in this volume.
I'm not entirely sure without reading it, but it's possible that no new major characters are introduced and the focus remains on Kazuma, Aqua, Megumin and Darkness.
One of the main plot points could be the further adventures of Kazuma and his party. Maybe they encounter new and strange monsters or face some difficult quests. Without reading it specifically, it's hard to say all the details.
I'm not entirely sure as I haven't read it recently. But generally, it might involve the further adventures of Kazuma and his party. They probably face new monsters and challenges in their strange world.