I understand the TYPE of game you are creating, and there was a time I did these myself. Mostly, TBH, I just found the ratio of work to fun is WAY high and zero prep worked even better. Hand over direction of where things go to the players, to a degree and just present obstacles.I run games like Emberashh and have run the same adventures for different groups and got totally different games.
Here is an example: pretend the "sandbox" is Western Europe, 1944. The GM has a plan that (barring player interference) addresses which way thr course of the war goes, which cities are leveled, etc.
Now....based on the players you can wind up with Saving Private Ryan, Inglorious Basterds or Overlord. Or one of a million other stories, which may, or may not, impact the arc of the setting.
I will generally lay out easy plot hooks that appeal to the characters based on the kind of game discussed in Session -1 (aka the "what game are we playing next" session) and Session 0. Inglorious Basterds get the French 5th column. Overlord gets drafted. Saving Private Ryan requires a bit more custom-crafted openings.
If midway through Ryan, they decide to pull a Basterds, OK. But Ryan could very well die. Or maybe they so disrupt the Reich that D-day doesn't happen.
Only time will tell.
I disagree. I don't think you've actually tried what @pemerton, or @Manbearcat do. I have yet to discover players who don't yearn to craft that story, to play fully fleshed out characters.Addendum: IME most players can't generate/spare the emotional energy for "driven" characters. They want a Forrest Gump game, where fate drops a metric ton of improbable opportunities to be "in the room where it happens" and their character can step up or just enjoy being in the background.