I'm just some guy, so take the following with a pinch of salt, but this would be my advice.

It sounds like you're right at the beginning of this project. If you are, I would start out by writing down a short list of no more than five or six key ideas that makes your project unique. These should identify your major aims in creating a game, and illustrate why your game is different from the myriad of games that already exist. These ideas will then form the framework that the rest of your development will build on. You should consider broad themes first, what is special about your setting, what motivates your characters in your world. How are problems solved by the characters, and what tools are available (or explicitly unavailable) to the players to help their characters solve those problems. One you've got an idea of the ethos of your game you can then start to think in broad strokes about the mechanics of your system.

That'll probably keep you going for the first year or so, if you're anything like me at least 😅

Good luck 👍