Mainly I'm thinking of the type of questions that pop up there every week: http://what-if.xkcd.com/
If not, which other SE site do you think it's appropriate for them?
If the question is literally just "What if X?" then no, it's not on topic here, or on any other SE site I can think of. A question like that is very open-ended; it's not asking anything specific, and it invites a whole slew of speculative answers.
Now, if you were to ask a specific question about an unusual situation, it should be fine. This week's what-if, for example,
What if you somehow managed to make a stereo travel at twice the speed of sound, would it sound backwards to someone who was just casually sitting somewhere as it flies by?
would be an acceptable question for us. But if you want to ask a question like that, make it clear that you've looked into it and done some prior research yourself, or at least thought about it. If you just posted the question as I quoted it above, it would be only so-so, but posting it as
What if you somehow managed to make a stereo travel at twice the speed of sound, would it sound backwards to someone who was just casually sitting somewhere as it flies by? I would think so, but then it occurred to me that blah blah blah blah so maybe it wouldn't. I can't find anything about this online.
(for example) makes it much better.
I would like to revisit this, with context to an Area 51 proposal which seems to be attractive.
The site proposal proposed by a user Krumia, is also based on XKCD What-If book.
The proposal is attractive, but I guess not enough to propose an entirely new SE site for the purpose. So I proposed Can we make “what-if” questions locally on-topic on their respective sites?
I also discussed this in the Physics chat room starting from this message.
I think we can have some support for such questions. Some specific ones which can be answered through little to no discussion but with supportive and established facts.
I would also like to see if the community is willing to accept questions like the ones in the Example Questions list of that proposal.
If all the three Science sites (Physics, Chemistry and Biology) agree to this, the proposal would be closed right here.