If you managed to get the salt mine to work with your setup then you’re already on a good path to creating your own mod 
I used Blender to create both the models and the textures (plus some basic image manipulation). If you want to do everything with Unity then that’s going to be difficult, if not impossible beyond the most basic geometries. Fortunately there are a lot of great tutorials for Blender on YouTube. A lot of people use separate software to create textures, but I didn’t want to spend that extra learning effort. There are also a lot of models available on the internet. You’ll have to check license information very carefully not to violate anything. Also these models need some adaption probably (size, texture format).
Roads are quite complicated if you want to do anything there. The speeds of roads is as follows (gleaned from source code):
| Road | speed |
|---|---|
| Side road | 20 |
| Gravel Road | 30 |
| Concrete road | 60 |
| Highway | 120 |
| Super Highway | 240 |
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These are realized with constants in the C# code. The problem with constants is that they can’t be changed with mods, because they are replaced with literals during compilation of the game. So what you have to do is to use a mod to patch every place where they are used in the original DLLs. Fortunately those places aren’t too many, I believe, but it’s hard to find all of them in the decompiled source.
I saw two specific projects that you want to realize, the roundabout the highway ramps. Both are going to be quite complicated. And to be honest, you don’t really need to create separate new structures for them. Roundabouts can already be created with the existing roads. Just making them circular is tricky. I had the idea of expanding the road construction menu to allow building round roads that behave like train track construction. That would make creating reasonable roundabouts with normal roads a decent task. As for the ramps, what you basically can do with existing tools, for example: create a 4 lane 2 way highway (so 2 lanes per direction). Connect a 1 lane 1 way highway to it as your ramp. From the intersection on, make the main highway 6 lane 2 way. Adapt the intersection settings and you basically have your ramp. The only things that are missing are convenient ways of constructing this and maybe the minimum angle between roads at intersections is too wide.
But don’t let me discourage you. If you have some cool ideas, then try to realize them. I’d recommend making as clear as possible a specification for yourself, so you have a picture of what you’re trying to achieve.
I’ll upload something later. Not in front of my computer for a couple more days. What you can do in the meantime is to subscribe to one of my mods and decompile the DLL to see how the script works. I’m not sure if it’s possible to decompile (or maybe rather „unzip“) the asset files.