Cool topic! That sounds very interesting.
I guess for me it kind of depends. Sometimes I’ll start with a character that I want to work with, and other times (like with my current WIP), I start with a what if such-and-such happened? type question, figure out a storyline that works, and deal with what remains whenever I can. I guess I don’t really have much of a specific system. . .
Od i lith naur echuidathar, calad od iñ gwaith twuiatha; megil i rangen cyrathar, alrin aran adolatha.