The Good Subjective/Bad Subjective guidelines talk a lot about what make for great questions and answers for subjective issues, and it primarily falls around Back It Up!: answers need to cite some kind of relevant material (scholarly works, authoritative discussions, relevant personal experience, something), and questions need to be written such that they encourage such answers.
And, of course, Back It Up! applies to objective questions and answers, too, the resources you use for backing those up just may be somewhat different.
So, we need to be pro-active, from the beginning, about demanding that answers Back It Up! This SE, in particular, I think, is closely tied to a lot of speculative fields – as several meta questions have already discussed, there are a number of SE sites that have significant overlap with this one, but would not accept our kind of question because of their speculative nature.
Thus, we need a policy, guideline, or at least discussion of what is too speculative, what questions cannot be answered in a way that Backs It Up! The “Primarily Opinion-Based” close reason exists for this, but we as a community have to decide where to draw the line for using it.
We also, as a community, need to enforce backing things up in answers. Everything about this endeavor is, by definition, highly speculative: the only way to be serious about this, to produce good answers, is to minimize, as much as possible, the “pure” speculation. Cite sources, compare to real-world history, sociology, physics, and so on. And flag or downvote answers that don’t.