Unit bonuses.

[ElorTeglen]

How about units can gain bonus from certain structure in civ or be trained as a specialized unit and gain bonus from that.

 

[The Last Conformist]

How about you explain that idea in some greater detail?

(My initial reaction is that it sounds like a micromanagement hassle, but you'll have to offer something a bit more concrete 'fore I can tell if that's fair.)

 

[ElorTeglen]

Simple Like if you build "U-Boat" factory as a wonder or just a simple building you get bonus for your submarines.

 

[Dell19]

How do these buildings affect older units that may pass through the city? What would these upgrades actually be?

 

[DogmaDog]

Huh....like building a barracks makes all your ground units come out as veterans instead of regulars???

DD

 

[Dell19]

I think he probably means on top of this so there wuld be more buildings which could provide an attack bonus or another bonus. Maybe a stable could be used to give units an extra movement point on roads or something similar. Not a full point bonus but a slight advantage...

 

[ElorTeglen]

Yup, another example would be like "Tactical center" Army gains +1 attack bonus.