Had a thought about unique improvements some ago (things like Odio's Prison, Remnants of Patria, et al).
The idea is whether it would be desirable to have a player's option to make those unique features that didn't get spawned at the beginning of the game buildable (like city wonders). IIRC a lot of these unique improvement are tied to the legends behind FfH, and as such they wouldn't be suitable, but some might lend themselves to this option. If not, then it might be a good idea to offer an additional set of unique improvements for Civs to be able to construct. First one who builds it gets it, as usual.
Such improvements might tie down a good number of workers, perhaps an engineer, or a mage for a long period of time, perhaps cost a certain amount of gold per turn as well. The unfinished feature can be destroyed (???).
This could include a super fortress, Moria underneath a mountain, a city of the dead, a Brigadoon-like city, etc...