Firstly I'd like to say this is a pretty nice starting point; this really only took 4 days to make? Honestly I don't know if I'm the best judge for deciding any kind of direction this game could go in, but I think the city roads themselves should become more complex in the future. More complex strings of intersections, cul-de-sacs, maybe a roundabout or two, maybe some alleyways to cut across, and the highway area definitely needs more entrance/exit points. Some sub-areas to explore might be a nice addition too. In a nutshell, the more distinct areas you can have in your city, the better, in my opinion. And then from there, maybe more ideas will come.