The first sentence of the site FAQ says:
Science Fiction and Fantasy - Stack Exchange is for questions targeted towards science fiction and fantasy enthusiasts.
To me, that should include speculative science questions such as this one (from the soon-to-be-closed Astronomy site):
So, if such questions are on-topic, shouldn't we also look through "speculative" tagged questions on the Astronomy site and ask for them to be migrated over?
P.S. Considering that most science fiction enthusiasts like astronomy, we could conceivably justify subsuming the entire site, or at least the best questions from it.