Fire Emblem: Three Houses' "Cindered Shadows" DLC lets you explore the Abyss, a sprawling town that's hidden underneath the Garreg Mach Monastery. In an interview with Nintendo Dream magazine, director Toshiyuki Kusakihara reveals that this location was originally considered for the main game, but it proved too big to handle.
“We started the actual production of the side story’s content much later, but we had wanted to incorporate the idea of Garreg Mach having this underground labyrinth since the early stages of development. It ended up being cut due to the sheer amount of content, though. When planning the DLC, the side story actually came from a desire to bring back the concepts we had originally omitted. It wasn’t necessarily that we were retrofitting the information. Rather, we were expanding upon the setting and some foreshadowed elements that had been there since production of the main game. The protagonist’s mother, for instance, would fall into that category.”