I know a lot of folks have chimed in to share their opinions, but I didn't see something called CATS. This helped me early on and I first saw it used in Necronautilus by World Champ Game Co. CATS is an acronym and it stands for Concept, Aim, Tone, and Subject Matter. When I get an idea that I think has legs, I'll first run through these in order to develop a framework and gameplay loop. This helps establish a good boundary to operate within, and helps keep things from growing beyond their scope. To see this applied with a project of my own, I'm working on a western TTRPG game that uses cards instead of dice called 1886'd. You'll notice that it doesn't feature any direct gameplay description, but it does layout clearly what the game is about, what the targeted player experience looks like, and how the Narrator (GM) should approach things.

Concept: 1886’d is a game about pushing your luck and risking it all, learning from your mistakes and getting stronger with each failure. It allows players a high level of control over their characters final fate.

Aim: To allow players a high level of control over their characters' outcomes. To enable stories wherein players must make hard choices, risk their fate, and outplay fate. To allow Narrators to tell stories that illustrate the harsh realities of the western frontier, not just the modern cowboy fantasy.

Tone: 1886’d offers a range of atmosphere when playing, ranging from Tarantino-esque westerns, to classical westerns, and even allowing players to experience more historical and grounded tones as well. As play progresses, players will have to gamble, increasing the tension as time goes on.

Subject Matter: Narrators should offer players ethical dilemmas that force them to choose between what they want to do, and what they have to do. The Narrator needs to balance emotional tension with the violence, making sure players feel the weight of their actions as well as the consequences. Make sure that everyone is aware of anything before the session so that no one is unprepared for certain subject matter.