Here is my tip this time: try to find a lot of anchors outside of the game engine. You can do design incrementally by defining what the "rules", "logic", and "lore" of the world are, in order to clarify it. For example, use a mind map tool like a writer to plan scenario ideas. Pick a genre of music and a visual design language and incorporate them into the game's identity. Consider the limitations of the camera and control scheme and allow those elements to shape the scenarios. Borrow plots and characters from stories you like. Compose an essay response to something you want to critique, and then figure out ways to turn that essay into game form. If you don't have ideas for game mechanics, you can develop the story more instead and you'll eventually find some story elements that convert into mechanics.
The more of these things you can piece together in a coherent way, the more it will start to look like a complete design where "the work is cut out for you", instead of being a mass of bad content to crank out. There are a lot of ways to go about defining the design, but you will have to find some anchors if you want good content without a lot of meandering and second-guessing. The meandering should happen in notes and sketches, well before you try to write final code or assets.
Oh, and all of this can be done with an eye towards marketing. You can opt to choose and highlight elements specifically to capture an audience. The hard part tends to be finding concepts that you're happy enough with to want to think about for months on end.
for example a "sticky" thread titled "Game design resources" from one of the subforums:
Also, I'd think people like to help with generating/brainstorming some ideas when provoked, like you're doing here with throwing around some thoughts. I can barely resist adding my $0.02 of random suggestions already :)
There is probably a name for this concept, but I don't know what it is.