Update: launched; see this meta post. Renamed to Scientific Speculation based on feedback here and there.
I'm copying this post from meta to keep the site suggestions together.
The scope of Worldbuilding.SE is very broad, ranging from rigorous questions about orbital mechanics to questions about the design of magical creatures. Some users from there have been discussing a more science-focused community here. It's still about worldbuilding, in the sense that questions are about things that don't necessarily exist in our world, but it'd focus more on science and things that can be reasonably extrapolated from current science than pure imagination in its speculative questions.
Clarification in response to this question: in some sense anything that doesn't exist yet is "magic". This includes faster-than-light travel, alien biology, creatures that don't exist, robots with human-level (or greater) intelligence, and so on. And most technology is previous generations' magic. These sorts of things are in scope. What we aim to exclude, because it's had some big challenges on Worldbuilding.SE, is stuff not based on real-world science or reasonable and well-defined extrapolations or extensions thereof. (This is partly pasted from a discussion that preceded this proposal, and I'll invite the participants to further clarify here.)
This community would want to import all open questions from Worldbuilding SE that are not tagged "magic". This might still bring along a little stuff we don't want and will have to clean up, but there are a lot of science-based tags there so specifying the positive list turned out to be verbose. I said "not closed", but if we could get dupes and have them be properly wired up here, that'd be great. If not, better to exclude them than have a bunch of disconnected dupes.
This community would use categories, including one special use we'll describe in an answer (does not require any new features). We don't anticipate using required or topic tags.
This community will need MathJax. A lot of the content we're importing already uses it and we need some sort of science-notation interface.