When it comes to designing the features and mechanics of a game, I believe in designing everything from the ground up with a purpose. What does the player need to see or learn when entering a particular area? How will a new enemy type change up the way a player plays? When would a player want to use one weapon or ability over another? I make sure to apply this approach consistently whenever I must take charge over a design.
Designing this way not only helps to keep a game's features consistent with its overall feel and aesthetic, but is also a helpful tool for controlling the moment to moment feel of a game and making sure every encounter makes the player do or feel something new. When everything is being finalized and it comes time to take features to the cutting room floor, this doctrine will simplify the process of deciding which features go and which remain since there will be greater clarity in how they all fit within the game's design space.
Any large project I work on has a design document, even if it is just a rough one and even if its just a solo project. This lets me keep all of my ideas in one place (instead of scattered in various notebooks) and makes it convenient to communicate these ideas with team members.
