Yeah, in mini-metro the roads aren't directional because the trains travel both ways! I could see developing this in the mini-metro direction.
Hm. Villages randomly popping up, with towers by them. Make a revolving track between villages to shuttle archers between towers, and then directed graphs from special towns/buildings to upgrade town buildings. Possibly get rid of the centralized producer.
Not sure if what I see is what your vision is, but I can definitely see how a game would be developed from that inspiration and this base :)