I'll agree about the unnecessary yet awesome nature of the feature. I think it would open up a lot of doors for extra interaction, but the game doesn't really need it. That being said, I'll continue to field questions and suggestions.
Yes, the player overseeing the Fortress would be the God. When they talk to you, it'd be to YOU, not to some predefined, procedurally generated entity. You, yourself, would set the spheres and traits. If you later contradicted those, you would be contradicting yourself. Which could be a cause for loyalty cascades and the like down the road. The game already has to deal with that on the small scale. Remember Adventure Mode? Well, now that characters have personalities, the players are constantly overriding their Adventurers' personalities.
And I'm not sure how a separate God mode would work. It could be a good mesh of Adventure and Fortress mode where you haunt someone, convert them by favoring them over their rivals, and then ordering them to build you a Fortress in the wilderness, but that seems like even more work just to do something that could be better done by allowing Adventurers to form their own entities and actively control the sites they claim or make.