... or you'll end up going in circles.
This is my take on the "generic system vs fixed setting".
Some vague definitions first. Game "systems" are games alright, only with a variety of possible settings. Games with a fixed setting you can play from very different in-game perspectives (situations, factions, characters). Picture a huis clos (locked in together) or a campfire in any setting : situation prevails. Some games like Lady B start with a very strict set of characters and situations, and many hacks of them are just skins are just really the same game with the house rule that you generate your own characters.
Originally I encountered a similar problem with "fantasy". Fantasy SF, fantasy magic, fantasy resources, economics and factions. You can start wherever and handwave whatever. And this is a problem for players that are neither preteens nor versed in storygames or improv. Open imaginary field is too wide, give them constraints and they'll set a planet in motion.
Also, I can bet there will be many misunderstandings between players unseen at that time that will fall on the road when the story is at full speed. And I don't want them to have to read an essay to have fun playing, right ?
So where do we start ? My compass is in the title. If I want to do character exploration, for instance, I can't start defining the character. So I've got to start the chain somewhere to "frame" my character question mark in.
I say "chain" because there are some heavy aesthetic decisions along the road (realism, power games, discovery, etc.) Each decision is reducible to a few prompts, and the chain to a handful of links, but :
1) players will only take that much decisions before it becomes an effort
2) my argument is that the order in which they approach these prompts is a decision in itself.
As for me, I want the most generic game for my players because I enjoy openness. But instead of having a rigid framework ala Gurps : world-characters-themes-situations, I want everything fixed in the RAW rules, and every table loosening the chain at any point they wish, feel how it reacts, nest their game in the resulting object, or link that to their existing system. So I start with everything fixed because nothing is, in the end.
Hope it makes some sense and sheds light on why I don't favour "chose your world" openness in storygames.