Earlier today I was explaining to the OP of a speculative what-if question why that kind of question is off-topic here and better suited to Worldbuilding SE. I like to support my arguments by links to meta or the help centre where possible, so I looked at /help/on-topic in the hope of finding something relevant there. But that page doesn't seem to say anything about Worldbuilding-type questions!
This is one of the few pages in the help centre which is editable by site mods, so the text there is specifically tailored to SFF. However, it was written long ago when Worldbuilding SE didn't exist. Perhaps back then it was thought that such questions were obviously a bad fit for SE; now that Worldbuilding has proved that thought wrong, maybe we need at least to state explicitly that SFF specifically still considers them off-topic.
Should we mention Worldbuilding-type questions in our on-topic page?
One main point to consider is how do we actually define such questions. Clearly something like "if I went back thirty years in time, how could I most quickly get rich" isn't on-topic here, but how do we define this type of question? Calling it a what-if question isn't good enough, because some what-if questions are fine ("what if Gandalf had claimed the Ring", for example, is answerable from canon). Calling it a non-franchise-specific question isn't good enough either, because the community is still divided on whether some non-franchise-specific questions should be on-topic.
The best I can think of at the moment is to put it in the third set of bullet points, something like:
If your question is about...
Ugh. That needs work.
What do you all think?