They had a Game Master mode in Divinty Original Sin 2. Was kind of like a campaign maker thing. You’d make some encounters and some interstitial cards with choices and skill checks and connect them all up. Still needed someone sitting in the GM seat to drive though, so you couldn’t just make a single player campaign for people to play by themselves.

I expect it’ll be at least a year before they have the game in a stable enough state that they might consider releasing some kind of mod support stuff, but I don’t see them putting out anything as user friendly as a GM mode. They promoted the GM mode feature pretty notably before the Divinity OS2 release so I’d think we’d have heard something by now if they intended to do something similar.