Even as I work on my fantasy tales, I am meditating on doing some space s-f. There's a whole bunch of different world-building questions with that genre. Like,
Is there faster-than-light travel? (the answer to this one affects the answer to the rest, since if there is no FTL, travel is confined to one system unless another system is quite close by)
Are civilizations common or rare?
Are there multiple civilizations, an overarching galactic "Federation" or "Empire", individual systems?
Are the aliens "alien" or just humans with pointy ears and extra bumps on their heads?
Is there lots of unknown space so that exploring that space is important (Star Trek) or is it a well-settled, well-traveled galaxy where conflicts, politics, etc. are more important (Star Wars)?
Probably more, but that's a start.