That was awesome idea! It should be not too hard, just adding some spells that grant building and make the building does not need the caster (so the building will not vanish as Hope and Inspiration spells).
Do you have any ideas on what building should be conjured by Amurite magic? I might add them as Wizard chanting spells for some spell spheres.
I'm not quite sure where to go with trying to move this idea from brain to actual gameplay mechanic.
I was thinking that it would be something that would require maintenance (making sure the spells have enough power, shoring up any magical threads that might have come loose over time, etc). But that would require some sort of special, low-level magic unit for the Amurites. I was also thinking about replacing workers with magic, but I don't see a way to do that in a way that wouldn't simply be a worker replacement with a different name.
Or maybe you can just assume that all buildings in Amurite territory have some sort of magical component and have a small chance to decay if no Mage-types are in the city? But that's more of a hindrance than a neat new feature. Is it possible for units stationed in a city to affect build times?
I'm kind of at a loss on this one. Maybe if someone else can throw out some suggestions one idea or another will stick that can be further elaborated.