[William]

"Unique" is really hard, given the number of people that have been speculating. But "new to your players" is probably doable! "Not often done in a fantasy setting" is also possible.

What do you want the megastructure to achieve? If it's just high-magic architecture that's huge, you're fairly unlimited in your choices.

The Counter-Orrery: To the circumference of this disc-shaped world, some unbelievably high-powered magical civilization has affixed an enormous ring of adamant and orichalcum, the creation of which alone would have required the plundering of multiple worlds of this size. The Counter-Orrery holds a cage which can be loaded with a city-sized patch of soil, ice, pitch and fuses, wood, or other material as needed. Driven by the fires of the Beneath and the Infinite Falls of the constantly-renewed world ocean drifting down into the void, this hoist can be raised above the world, putting the material into the desired position in the sky, where it modulates astrological influences on the fates of the people below.