I think you’re right on the concrete, established details working better than freeform ones. Like, when Jet Li walks into any room, the audience immediately looks around to see what he’s going to end up using to beat the shit out of people.

You know how, in Geiger Counter, I place little groups of dice on the map, for the players to pick up and use as weapons? Something like that might work, even if not everything about the weapons was defined before the fight started.