I do think it would be a big mistake for SCS to skip US-385 from US-16 to Custer at the bare minimum. Yes, SD-79 is more generally useful for trucks, but it skirts the Black Hills and doesn't go through it at all. Lots of people and businesses and even industries like logging, farming, and mining are in the Black Hills, so trucking is reasonably important there. Not a Yellowstone situation in the slightest.
In my opinion, the solid red roads are required, and the light red roads are all the different options SCS has to supplement the required roads. With regards to Wyoming, US-85 from Newcastle to Lusk would open up the eastern portion of the state and benefit WY the most. Continuing WY-450 from Black Thunder Coal Mine would be an easy addition that would give us new routes to take from the mine to SD. US-18 and SD-79 are the bypass option but like I've said I think that's a bad idea to only do those. As supplements for US-385/US-16 through the Black Hills, it's fine. But I'd like to see US-385 all the way to Deadwood to meet up with the solid red portion I have there to get to Wharf Mine (and pass by the defunct Homestake Mine, the former largest gold mine in the world). US-85 from Newcastle to Deadwood is another option that gives us more options as well. I am skeptical that there will be room for the entire SD-244 so I picked the eastern side of it but the western side could be used as well. The problem with the Black Hills are that hills/mountains take up lots of room, but SCS can use all that space in a creative way I'm sure.
If I missed an option let me know but I think I covered all the bases in the greater Black Hills area.
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For Wyoming, it would be nice to get something in the eastern portion of the state before SD comes. Maybe they can use newbies like Iberia did. Extending WY-450 and bringing US-16 to meet it would be a good start but would only be useful if Newcastle got a depot (marked to Gillette or marked as its own town). I'm not holding my breath on additions to eastern Wyoming before SD though. It'd be nice but I am not sure it will be seen as a priority by SCS.