Sometimes, it helps to look at your game in a structured way. These are some ways of thinking about your game that I've found useful. They're basically a way to focus your thoughts on one specific aspect of the game at a time. When I'm working on a game and I find myself stuck, I will sometimes turn to one of these tools to focus my thoughts and reflect on it to see if that helps clarify things. Sometimes it helps. (Sometimes it doesn't.)

This worksheet starts you thinking about how to allocate resources, and what is important for your game. 

The Power 19 is a list of questions to ask yourself about your game. It's a way to make sure you're thinking about different aspects of it, and how they all tie together to reinforce what your game is about. 

Vince Baker suggests that all games necessarily make statements about different topics: about your fictional subject matter, about roleplaying as a practice, and about human nature. So it's useful to ask yourself what your game is saying about each of those. What insights is your game built around? Is it saying what you want to say about each?

The Art of Game Design: A Book of Lenses by Jesse Shell is a book structured as a series of thought experiments, to help you think through different aspects of your game design. It's primarily aimed at videogames (the industry Shell works in) but has lessons applicable to tabletop games as well. (There's a free app for the lenses, which is more useful if you've read the book.)

Each of these has underlying assumptions that might not apply to your project (e.g., the worksheet's Allocate Authorities section doesn't make sense in a GMless or single player game, many lenses in Schell's book don't apply to analog RPGs.) But they still can be useful mental tools for thinking about a game. Even realizing that your game doesn't fit the author's assumptions is a significant thing to know about your game. Just don't let yourself fall into the trap of taking the author's assumptions for granted. That would lead to less creative games, when these tools are supposed to help you create more creative games.

Maybe you have some other similar frameworks for thinking about game design? I'd be interested to  see what you use to structure your thoughts.

Here's another example, from the RPG Design Panelcast. As with the others, it's useful to structure your thoughts about your design and how it hits its goals, but you should also think critically about the design assumptions baked into the questionnaire itself.