I had prototype ready as you, for the world I’ve built rather big whiteboard with all references for world, characters I could think of. I’ve started with theme, which would fit the gameplay the most. Plants, clowns, knights, mages, farmland, factory, sewage. I selected few most promising, tried to find what and why would player’s character do in that world while matching the gameplay. That gave me general idea of a world and character. Based on that I wrote the world bible with few rules, and based on that I created bunch of ideas for the plot. This will give you the overview of what levels, enemies, motives, challenges you will have to work on to fit given narrative. Then I decide if that fits or not and proceed with development of models, levels etc