That's really neat. Seems to have produced good play.
I think I may have gone from idle curiosity to starting to hunt down a suitable system and talking to some friends about trying it out.
I'm maybe thinking/ playing in terms of a different kind of black box than presented by the sandboxy, rules as light as possible to arrive at improvisation approach.
The rules are very much there, but the types of rules chosen are scalable systems that allow the GM, and players, to in effect zoom in and out as play goes on. If improvisation/ imaginative input is flowing freely the rules sit in the background and play is driven by the players' choices based on a loose framework. However, the GM and players can seamlessly draw on greater levels of detail/ zoom in on the rules to feed back into rekindling player-driven narratives.
So it's not an either or situation where wafer-thin rules leave the GM and players to constantly drive play - and it doesn't rely on random generation, as players choices and the situations they interact with form shared expectations which suggest which options present themselves.
Essentially, this is about leaving gaps, but not huge holes, for the GM and players' imaginations to fill - and calling upon the rules cavalry to support them in building further shared expectations alongside further improvisation.
The choice of rules set makes a difference here, as any RPG could be adapted to such purposes but that leaves a large 'transactional' overhead for GMs to mediate, i.e. is tiring, hard work.
Systems with certain characteristics make it very easy or fluid to zoom/ scale shared expectations and player choice, while balancing those expectations and choices.
Classic Traveller is the obvious example, as it's highly modular, easily extensible and everything connects up. For fantasy, Treasure was designed specifically to learn from Traveller with the intention of supporting this kind of play.
Improvisational jazz and jamming offer an analogy. There's a framework provided by the instruments, the musicians' skills and experience, the interaction between the musicians and the audience's interpretation, (and reaction), to the performance . . . However, improvisation and experimentation bobbles along on top of all that, often taking the lead/ shaping further improvisation, because the shared expectations formed by the context provide a stage or platform for such improvisation.