Diablo's design could be split into three main categories.
Story and quest lines: Building the foundations for the story as well as laying out every single quest line, allowing players to experience the story of the game seamlessly.
Map and level design: Create the scenes according to the story. Each chapter was placed in a different setting with different models and textures.
Moreover, one of Diablo's biggest pros were its dynamically generated maps. Every scene the player experiences are randomly generated. Elements such as dungeon entrances and altars are randomly generated every time, which makes exploration one of the important pleasures of the Diablo series.
Combat system and monsters: This focused on the abilities the players have in combat as well as the characteristics each monster had, making the combat system feel more rich.
After these three points are done, everything else could be polished slowly over time.
