TL;DR
A simple percentage-based HP boost for buildings that are constructed on pavement, the better the pavement (foundation), the bigger the bonus.What ?
I suggest adding a bonus to building HP based on what kind of foundation it's built on. Obviously raw terrain is 100% net HP (though different terrains might have a bonus or malus as well, as it would likely be easy to implement once the rest is).Something like...
Stone Path = 120%
Concrete = 150%
Refined Concrete = 200%
Additionally (and optionall), walls would get:
Stone Path = 120%
Concrete = 200%
Refined Concrete = 300%
Why ?
Right now, the primary use for pathways is for, well... paths. However, big endgame factories tend to be paved en masse for cosmetic reasons. This is something I enjoy doing myself, and I've seen others do it as well.Since paving large areas of land has a detrimental effect (no absorption of pollution), a bonus would be nice to compensate (better building HP). This is a bonus that isn't particularly OP, as it has no effect on build speed, but it still incentivizes eventually converting your factory to a concrete foundation.
For walls and defensive emplacements it would be very important, and much more dramatic, which makes a lot of sense. Build a wall on grass and it can be knocked over more easily, burrowed underneath, etc. Build a wall on a concrete foundation and it's much more sturdy. Build a stone wall on a cobblestone foundation, however, and it doesn't gain all that much, hence the equal bonus for it to normal buildings.
This should be fairly easy to implement, and if done the right way (setting a flag when a building is built, or a pathway is build underneath one), it should have minimal impact on performance.
