Terrain improvements should be limitless and tweakable, a lot. Maybe, if you need a tech to have a certain improvement, when a civ that doesn't have said tech conquers that tile then it wouldn't work for them. For example, Radar towers without {Radar technology}. Or Minefields without {Mines}.Or have a kind of "graphic override".
Example of how it's down in Napoleon Total War
By default, the game engine look for a graphic file like
"Irrigation.png"
Then, if you have a civilization, add the name of the civilization as a prefix/suffix.
Exemple if you play has Britain and the engine can find "Irrigation_Britain.png", then it uses this specific graphic.
If you play as France, and there is no "Irrigation_France.png", then it uses the default graphics.
This way, in the rules, you can enter irrigation only once, but the graphics can be easily made specific to a civilization/culture with no rule changes.
Of you can allow in the rules to have different improvment, each with different bonuses, and then tell what civilization are allowed to build it.
Or have a generic improvment, but give bonus to each civilization, like "agricutural" gives a bonus to farms...
And fo course you can have different improvment levels with different techs...
So many possibilities
Also, several techs could be allowed for the same worker action/improvement to avoid overdependence and duplication of techs.
Or we could have more than one tech requried for a tech/building/unit.
Would be trickyI like Steph's idea too. Nice way to maximize graphic choices with minimal necessary input. I hope Steph is looking for ways to incorporate some (any) of these ideas into his editor as well...