I would really love to see, in future versions of Unity (Unity 6.3 or Unity 7), a built-in road system integrated directly into the engine – built on top of the current Spline package and terrain system. 
The Spline package itself is very good: you can create camera/object animations, deform or generate meshes (such as tubes or ropes), and instantiate geometry along a spline for things like fences, trees, etc. But what it really lacks the most is integrated support for Unity’s terrain system. Right now, you can’t create roads with it out of the box. Of course, with its API, it’s possible to write custom scripts to generate a road mesh along a spline and deform the terrain to follow it. Still, I’d really prefer Unity to offer an official tool for creating roads.
I believe this road system should be capable of:
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Creating roads directly on terrain, where terrain and splines work seamlessly together;
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Automatically generating roadside elements such as sidewalks, guardrails, fences, streetlights, traffic signs;
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Supporting complex shapes like roundabouts, bridges, and intersections;
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Using custom materials with shaders that remove texture tiling/repetition;
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Applying road markings (dashed lines, solid lines, pedestrian crossings);
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Allowing customization of road width, number of lanes, and also the shape and size of sidewalks;
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Supporting off-road paths (for rural or forest roads);
Here are some existing asset examples:
Also, I discovered something very interesting!
Apparently, there’s a sample scene in HDRP called “Test Track”, created in collaboration with Volvo. This sample includes a car controller script, a scene with a Volvo model, scattering tools to dress terrain, optimization tools, but most importantly – a basic road tool. I don’t understand why Unity didn’t continue developing this package into a proper road system. From what I can see, it’s already a solid starting point with real potential:
I would really love to see these improvements in future Unity releases and in the Spline package (maybe in version 3.0.0 or Unity 7).
I really like Unity 
– I work with this engine with great enthusiasm and I’m proud of what has been introduced in Unity 6.0, 6.1, and 6.2, but I’d love to see just a bit more effort put into these tools. I’m not asking for Unity to become like Unreal or even surpass it; I just hope for small improvements that would make a big difference for us and many other users.
This feedback comes from the context of my game, which will include several cities and will really benefit from such a system.