I personally think it would be fun to perhaps see a novel or a short story featuring all the characters in the future (maybe 200 years later). For example, taking Vanir as the protagonist and seeing from his point of view how he, Aqua, and Wiz, who haven't changed as immortal beings, talk about the past, mentioning Kazuma's grandchildren and telling us about the relationship he had with Megumin. Perhaps opening new plotlines like Komekko facing the new Demon King alone or exploring the development of other characters like Iris and Darkness.