Hide unique and regional stuff from civilizations for which they are unavailable

[HolyRomanEmperor]

Hide unique and optionally regional units and buildings from civilization tech trees for which the unit or building is unavailable. For example, hiding Fire Lancers for Hindustanis and Folwark for Franks.

[HSZemi]

I think going for fully individual tech tree layouts (as the game does) would be preferable here. A WIP implementation is on the civ-tech-trees-only branch