This reminds me of two things.

1. The Civil War campaign for Marvel Heroic, which I suppose is not really sandbox-y at all, given that it’s a linear set of scenes with a little bit of branching. But! When we played through we had two PCs (sometimes three, depending on attendance) that were not Marvel characters, and some additional scenes grew organically out of play. In the end, it was simply an alternate-universe version of Marvel’s event, which I guess was the intent, but it felt like there was actually a lot of leeway in how the characters experienced the event, despite the fact that specific scenes were going to happen no matter what. So, maybe on-topic or not.

2. I’ve had short conversations with Ron Edwards (plus him in if you want, Paul) about how he feels that sandboxes don’t even really exist. IIRC, meaning that the players’ choices are still limited and certain stuff prepped by the GM is going to happen, just in a yet-to-be-specified order. I’ll admit I’m a little fuzzy on his reasoning, but it came to mind, because I think it’s making the point that either “story” is emergent or it’s not, and maybe “sandbox but there is a plot” is genuinely an impossible breakfast thing.