The problem a lot of sandbox, living worlds people have with settling on a core play loop is it seems to be overly reductive and potentially could lead to more constrained play. But I think most sandbox GMs agree the fundamental exchange in play is: player declares what they want to do, know, see, etc; and the GM determines what the result is (often by invoking rules, formulating rulings, or declaring based on what seems most reasonable/exciting/etc). But the exchange is a lot more varied and organic than that breakdown suggests. I think players wandering into a such a session with that set of steps loaded into brain, will actually have more trouble navigating what is going on, because the exchange isn't that A to B all the time. Also a lot is unspoken social dynamic. That is very hard to pin down
It is.
But my suggestion (same as it has always been) is that we get better at it (pinning it down).
For instance, this might be how one would describe the play loop of a social conflict in a GM Curated, Interactive Sandbox, Skilled Play game:
SOCIAL CONFLICT
1) The GM rolls on the NPC Reaction Table for the primary NPC, taking any adjustment for PC Charisma Score or present/past PC activity. Example:
+2 for lead PC
- 1 for past transgressions against NPC motivations
+1 for decorum in arrangement of meeting.
+ 2 total.
2d6 + 2 = 9. Hospitable. NPC considers offer.
2) GM presents the situation which includes the setting, the relevant NPCs, and the depiction of their orientation of the PC based on the NPC Reaction Roll.
GM roleplays through the NPC, using their motivations, and presents the PCs with the NPC's position on the subject matter at hand. This should be something to either put the PCs on the defensive or provoke the players to present their own viewpoint through their PCs.
3) This back-and-forth continues (with a player possibly declaring an action that forces dice to be rolled - History or Insight to develop rapport for instance) until the GM decides that either the players has made the case and the NPC agrees (ending the social conflict), hasn't made the case and the NPC rebuffs them (ending the social conflict), or the GM is unsure if the case has been made. The GM then sets a DC (with the NPCs orientation as a base and adjusted for the factors of the NPC Reaction Roll and any transgression or well-made point in the course of conversation) and the player then makes a Charisma roll. If this Charisma roll meets or exceeds the DC, the NPC agrees to the PC proposition. If the Charisma roll does not meet or exceed the DC, the NPC rebuffs the PCs.
That expresses the elements of the play priorities/style pretty clearly.
GM Curated (the GM created this NPC and the setting)
Interactive Sandbox (the players interact with NPCs and the NPCs respond based on their motivations and resolution mechanics)
Skilled Play (the players will be more apt to have success if they build their PCs correctly and declare actions/converse skillfully in the social conflict)