I think there may be more to why we're not getting this mode... and I don't believe for a second that there is a technical hurdle that can't be overcome fairly easily. This is, after all, a Divinity engine game.
I bet it has something to do with the licensing and content gate-keeping. If WotC was going to license world/campaign building, it would probably be a separate license and require more adherence to the game system. And, frankly, with D&D One coming out there would be no good reason for them to license an application based on the old system. As for Larian, they have more incentive to gate-keep content creation for DLC and the inevitable BG 4+. There just isn't much of a profit motive for either to want a capable DM mode out there.
