I think the two scenarios identified in the close reason text are a good starting place.
There are too many possible answers
The most obvious examples of this are "list" questions. In fact, the accepted answer from that linked discussion contains a very helpful definition for "too broad":
It's not reasonable to ask a question on a Q&A site that requires knowledge of all works that ever existed
"How do you kill a vampire?" (in the absence of clarifying details) is essentially an open-ended list question.
Note that something like "are there any examples of vampires being killed by explosion" would be considered on topic (although this is a policy I don't agree with).
However, with slight rewording, open ended list questions can be reworded to asking for common tropes, which I think would be acceptable. "What are the most common ways in which vampires can be killed?" becomes rather more narrowly scoped. It is still a list, but I think it is a list that someone expert in the genre can provide a reasonably thorough explanation for. If an answer citing half a dozen examples would satisfy the wording of the question, then it is reasonably scoped.
Good answers would be too long for this format
This one is trickier for our site. We have a couple of users here who sometimes post very long, detailed, in-depth answers that would be nearly unheard of on most other SE sites. Those answers also tend to be my favorites on this site.
For example, this detailed exposition on the workings, history and variations of vibranium.
I'd hate to see any policy that discourages questions soliciting answers like that.
So for our purposes, I think "good answers would be too long for this format" should largely overlap with "unclear what you're asking", when we're not specifically dealing with list questions.
For example, "Why are vampires so powerful?" could theoretically be answered by in-depth descriptions of all the various ways that vampires are portrayed as having power (or even half a dozen examples), but is the OP looking at vampires as being powerful because of physical strength? Or their ability to decimate entire populations? Or are they referring to magic-using vampires from specific settings (such as Strahd)?
The question needs clarification. What type of power is the OP interested in?
An example listed in another post of a "terrible" answer that is justification for why questions on scifi tropes in general (in this case, creature tropes) should be closed is actually what I consider an example of exactly the opposite: it is, at the time of this writing, sitting at 41 upvotes, and 0 downvotes. Clearly the community has an appreciation for this type of content, and the opponents, while vocal and active members of the community, are a minority.
I believe that "too broad" for our site should cover list questions, and questions that are asking for too much detail about too many diverse topics.
Outside of list questions, "too broad" should be used very sparingly, and the OP of any question marked as "too broad" should be encouraged to be more specific with the information they're seeking.