When you start designing a new game you have keep in mind very clearly WHY you are building the game. To help in this process I have developed a Game Vision Board. This is directly derived from Product Vision Board by Roman Pichler and adapted to specific context of game design.
This canvas is composed by 5 areas where creators put main attributes of the game and, at the end, the Vision statement that will guide in all decisions. These attributes are:
1) TARGET: who are players? (think of personas, how many, interactions, …)
2) NEEDS: why players want to play it? (think of what is motivating players to play it)
3) FEATURE: what makes the game stand out? (think of unique/new and imported/improved mechanics, components,… you would like to have)
4) GOAL: why this is useful for us? (think of what is motivating us to build it)
4+) How do we MEASURE success? (think of some metrics and how to measure them)
5) VISION in a sentence (think of a tweet)

FOR EXAMPLE recently I worked with a team of serious game designer about a possible new game on sustainability. We spent a couple of sessions to understand and share our ideas around this game and we put everything in the canvas below.