Write down your design pillars. This is the foundation of your game and should never change. Your whole design should tie back to these pillars.
For example:
Precise Aiming/Shooting
Specific body parts deal less/more damage.
Real Physics
Physics need to be on par with most 3D games.
True Western
Being true to the western genre
<aside> 📚 What are design pillars? https://www.charliecleveland.com/game-pillars/
</aside>