here are the limit i would have put to the strategic ressources:
- limit in production: you will be limited in the number of unit you can build at the same time.
for example: if you have 3 iron, you can:
A) train 1 swordman if you got no encampment by using 2 iron.
B) train 2 swordmen if you have 1 encampment by using 3 iron. (or if you have 2 encampment by using 2 iron.)
C) train 3 swordmen if you have 3 encampment by using 3 iron.
NB: in current version of civ 6, if you got 2 iron you can build swordmen in all of your cities.
- limit production speed based on excess strategic ressources : the more unit you have that are requiring a specific ressource, the more expensive next unit would be.
the math would be like : price of the unit * (1-((number of StrategicRessources - number of unit). 0.02)
that would mean that strategic unit (unit requiring strategic ressources) would be easier to get when you have extra strategic ressources. (having 4iron and 2 swordmen would mean that next swordman would cost 96% of it's initial price.
that would also mean that building strategic unit beyond your capacity to supply them would require more time, making some soft limit to the number you can build with a given amount of strategic ressources. (that would, at some point, make the non-strategic unit more attractive)
- heal based on the amount of strategic supply you have.
that would be similar to the previous rule: if you have more strategic unit than strategic ressources those strategic unit would suffer from attrition.
for example:
if you have 4 iron and 6 injured swordmen, then each swordmen would heal at 2/3 the initial speed.
NB: noted that it count only those injured, you could have more than 6 swordmen.
that would make it harder to keep an army full of strategic unit when you are beyond your capacity, making a mix of strategic/ non strategic unit switching position more affordable as they would heal faster.