I want to ask about game design for a totally homebrew game I'm doing. But having read the rules it appears the questions are specifically known tabletops. Is asking about how I can design my game against the rules? If so, any suggestions for where else I should look?
Game design questions, broadly speaking, are on-topic. See game-design, and also our encounter-design, campaign-design, homebrew, and tags-about-design-im-forgetting tags. But they still need to be good Stack-answerable questions, which are sometimes tricky in the world of design.
Too broad and unclear are likely tripping points. One key when writing questions is to go ahead and be selfish: ask a very specific question. You don't have to worry about making the question too pointed: good answers will tend to expound design principles and processes that future readers can leverage for their own uses.
Idea-generation is not going to go well. "I want to have ten distinctive tribes of goblins, what should their dress look like?" is the sort of thing better suited to a forum. Sometimes those questions get re-phrased as "how do I go about designing ten distinctive tribes' garb?" which is passable, but (in my opinion) don't tend to get great answers. If what you're stuck on is coming up with names for goblin tribes, just pop into one of those fora linked above and ask. Not every bit of your homebrew process will fit the stack model.
When you're not sure go ahead and make use of our resources. As HeyICanChan mentioned in a comment it's always fine to post a "first draft" of a question here on meta to ask if it's going to work on mainsite. [Chat] is also a helpful resource, usually when someone's looking for a vaguer "will asking about this topic work?" up/down.
Lastly, remember that if down- or close-votes do come the way of any of your eventual questions please remember: they're not a reflection on you, they're a reflection on the post. I hope to see some posts about your game start dribbling in and hope you, too, find RPGSE to be a healthy community full of knowledgeable, considerate people.