Scoping practices
The best way to approach scoping is by deconstructing the game from the top and defining the critical progression path of the player. Think of the final experience you want to deliver or the story you want to tell. Thinking about levels or any other units of player progress, can you define the minimal, optimal, and nice to have quantities? How much new content (obstacles, NPCs, game mechanics, and so on) you'll need on each part of the critical path to keep things interesting? By dividing player experience into chunks, you'll be able to easily estimate the amount of all interdependent elements.
Content lifespan
The content lifespan is a document that lists every significant piece of content and maps it against a player's journey in the game. It allows people to plan the production, estimate which elements are needed first, and identify the areas that have too many or too few new elements.
Unless you're working on a very open-ended game or a sandbox experience, it should be...
