Personally I like building in Minecraft a lot, and What topics can I ask about here? is unclear on whether asking for ideas for designing (maybe it falls on the first bullet?), and so far I haven't seen such questions. So are these kind of question on-topic?
No, they are not. It's because the questions are totally subjective; there's no way to judge the value of an answer except by what the reader personally likes, making it a popularity contest. It's basically a vague, unbound list.
The only way this would work is if you had a specific, concrete build in mind, such as a cannon, and you were missing something that would make it work. "Why doesn't it fire?" That's objective, and answers just have to solve the narrow, focused problem.