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.
A Konosuba - Re:Zero light novel crossover might explore the different magic systems. In Konosuba, magic is often misused or has unexpected results, while in Re:Zero, magic has a more serious and often dangerous undertone. The crossover could play with how the characters from each series view and interact with the other's magic. Also, the settings are quite different. Konosuba has a more light - hearted and comical fantasy world, while Re:Zero's world is darker and full of political intrigue. Combining these two settings could create a really unique story.
One possible plot could be that a strange magical anomaly causes the two worlds to collide. Kazuma's group and Subaru are suddenly aware of the existence of the other world. They initially misunderstand each other and get into comical conflicts. But as they start to understand the situation, they team up to find a way to separate the two worlds again. Along the way, they face challenges that combine the elements of both worlds, like the Re:Zero monsters with the Konosuba - style humor.
A possible crossover could be in the form of a cooking competition. Since both series have characters who are somewhat inept at normal, everyday tasks like cooking. Emilia from 'Konosuba' and Emi Yusa from 'The Devil is a Part - Timer' could end up in a cooking battle where they both struggle to make a decent meal. The chaos that ensues with their respective friends trying to help (or hinder) would be a great aspect of this crossover in the fanfic.
Emilia from 'Konosuba' is a bit naive and innocent. When paired with Emi Yusa from 'The Devil is a Part - Timer' who is strong - willed and a bit of a hothead, it creates an interesting dynamic. Emilia could mellow out Emi's temper, while Emi could teach Emilia to be more assertive. Their different personalities would blend in a way that changes both of them for the better in the fanfic.