Hyrule Warriors: Age of Calamity was the spin-off that no one saw coming. The original Hyrule Warriors was a surprise, but we still didn't see a pseudo-sequel in the pipeline. How did Nintendo decide to team with Koei Tecmo for this follow-up? Eiji Aonuma shared some insight in an interview with Game Informer.
We weren't able to depict the Great Calamity in BotW, but [Breath of the Wild director Hidemaro] Fujibayashi-san wanted to find a way to bring those events to life. Development on this project started when he got together with Hayashi-san from Koei Tecmo Games, who was interested in bringing the experience gained from working on Hyrule Warriors into a subsequent project, and we recognized the value of creating this new game.