What you call "fishing-for-ideas" questions have been known on this site as ideas-generation questions. You can go and do a search on WB Meta to see how they have been dealt with the past. Previously, ideas-generation was considered a reason to vote to close, however, this was replaced with the VTC categories of opinion-based and too broad and is no longer reason to vote for closure in itself.
What makes any question off-topic is the fact the question itself is not about worldbuilding. An idea-generation or "fishing-for-ideas" question can be on-topic in the sense that it is concerned with worldbuilding. However, they will usually be closed as opinion-based or too broad.
The Stack Exchange model has deliberately excluded questions about ideas-exchange. Essentially because it is deliberately interested in cultivating questions of useful and practical kind about software and hardware which is where its money is. "Ideas-exchange" seems to be interpreted here on WB SE as "idea-generation" although I haven't been able to find anywhere where this notion originated. Hopefully wiser heads will provide a link.
This does lead to an interesting paradox. A reasonable number of Stack Exchange are most definitely ideas-exchanges, for example, the Physics SE and, of course, Worldbuilding SE itself.
Recommendation 1: All further discussion should avoid using the term "fishing-for-ideas" as this creates the false impression that there is a new category of questions liable for closure. Let's stick with "idea-generation' for the sake of clarity and good sense.
The OP has identified what he calls ' "well-defined fishing-for-ideas questions."' These are: --
The user has constrained the question well enough that the ideas, despite being primarily opinions (which is what creativity basically is), can be well enough focused for the OP to reasonably select the one that is higher quality than the others.
This is basically what WB SE constrains querents to do. To ask questions about focused ideas that participants can provide an answer. I have ignored the notion that creativity is primarily opinions and the confusing proposition about how the OP had select the quality of one over others.
Basically if a question is focused in a way that enables it to be answered, then it conforms to the Stack Exchange model.
In its worst case, I suspect the off-topic fishing-for-ideas question is nothing more than an effort to overcome writer's block.
This is an entirely irrelevant proposition. There is absolutely no reason to ask someone is asking a question on WB SE. That is entirely their own business. Questions should be taken on their merits. Even the most frivolous question might be an essential part of worldbuilding in a work of fiction, game or TV script.
From my experience, idea-generation is a useful tool to explore the possibilities and consequences of a new worldbuilding concept. Sometimes only by exploring the aspects and facets of a concept can a worldbuilder come to grips with its possibilities. The hard thing can be knowing where to begin and only by getting feedback from others can be accomplished.
What elements or aspects of a fishing-for-ideas question makes is an on-topic and high-quality question?
The same elements or aspects as any other on-topic and high-quality question here. Namely, that it is about worldbuilding, that the question can be answered, that the question is well-written, often clearly defined, and is on a topic that can be dealt with by the expertise of participants (side-note: certain topics cannot be answered here due to a lack of expertise by participants and often are labelled opinion-based or too broad).
This site already has a well-established label of idea-generation for those questions of the type that concern you. The criteria for their vote to close exists in the form of opinion-based and too broad (they have their weaknesses, but that's another matter).
If a question about worldbuilding evades the criteria of opinion-based or too broad, then it is de facto within the boundaries of acceptable questions. (NB: I avoided calling them "on-topic" because there are different criteria about whether is on-topic.)
The main problem with this question is that conflates several separate concepts into new categories. This liable to create confusion rather than clarity. By inventing "fishing-for-ideas" when "idea-generation" already exists, and reinterpreting the criteria for on-topic and off-topic.
This kind of misconception could only be created by someone who is intelligent and thoughtful and is working with conceptually poor materials. This is absolutely compounded by difficulties in accessing the criteria for closure. There is NO easily accessible location for the criteria for closure on Worldbuilding SE. I know because almost on every occasion I have gone looking for them I only find more by accident than design. They should be easy to find on Help and accessible with a minimal numbers of clicks. This should be remedied as soon as possible.
Despite the fact that I disagree with the majority of propositions in this Meta post I do respect the honesty and reasonableness with which the poster has made his case. Any errors he has fallen into are due to the morass of misinformation and misdirection about locating the criteria for asking questions here and the criteria for voting to close.
Any disagreeableness on my part can be put down to my history as a scientist, a several composer of constitutions for various clubs and societies, and over twenty years working in the legal advice section of a government department. Never underestimate the wrath of a professional. On one hand, I applaud it when people try to do things I have had training and experience in doing, it's just when they use a hammer to drive in screws. Enough said.