Not sure how I would go about modding it in the first place. I would have to do quite a bit experimenting with the leveling of the roads. Not to mention I would probably have to draw up some new graphics to see if the game actually can detect new roads. Heavenfall might know more about this stuff, but I don't think he played with roads either.
I'm pretty sure that I cannot do your suggestion to say build roads between city A and B, that would be a feature request. I think it might be possible to change the type of roads to outposts... but I'm not sure about this either. I must experiment and decide exactly what I can accomplish with it.
My request was mostly tongue and cheek. I know that redesigned the road system in FE would be quite the feat to accomplish and most certainly would not expect someone to persue it out of obligation.
Graphics is a minor issue as the UI could be updated with new images after the mechanic was in place.
Designing a 'building' however that has zero Zone of Conrol connected with it may be easy, but this 'building' would also have to be serpated from the city so that you cannot just jump from city to city to city in one move.
The minimum distance between city buildings would also have to be considered, as this 'building' touches from city to city
There would also need to be a way to connect the different tiles in a way that generates the segment, for cost and build time, as well as upgrades and movement across those tiles.
The build time of this 'building' would be dynamic depending of length....not sure how that would work as you would need to select your 'sink' with the build.
Over all, I agree with you....quite the mod...mostlike this does indeed fall within the realm of 'feature request', but I hope it's a request that does make roads for the better and SD acts on it...