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 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.
I'm not entirely sure, but it's very likely. TvTropes is a comprehensive database for all kinds of fictional works. Given Konosuba's popularity, especially in the anime and light novel world, it would make sense for there to be a section about its visual novel on TvTropes. You could check by searching for 'Konosuba visual novel' on the TvTropes website.