Look for Briefs.
All I know about writing (is it III?)
Look for briefs.
The blank page is a terrible thing, but a quick look at various online resources (Ralan, Duotrope, whatever) that list open anthologies looking for material will often show up venues that are looking to collect stories around a particular theme. Straight away the writer has somewhere to start — a constraint that offers structure and directs thought. (And if the antho takes reprints, the writer may have something in the shelf they can send, too.) There are also some magazines that focus more narrowly than others, or have a mission or mandate. These include (a very non-exhaustive list):
It’s always possible to arbitrarily pick a topic, but the advantage of working to a brief is that it is set externally and fixed, and it has a deadline, both good things to work within to develop discipline. And unless the remit is extremely narrow, the story should be submittable elsewhere if it does not make the cut. And it can be submitted elsewhere for one very simple reason — it exists.
Anyway, anything that gives a jumping-off point has its uses.