Ayuh!

@Sir Ocean

So I personally imagined them as a culture (or maybe great person) civ

UA: Mount Rushmore (Cities settled adjacent to mountains gain a free artist specialist and a great work of art slot, all cultural specialists provide production)
UB: Something involving wall drug, I'd personally go with a Wall Drug billboard, as a caravansary replacement, with some tourism bonuses
UU: It doesn't work perfectly has a unique unit, as it's more of a organization, but maybe a Western South Dakota Buckaroo Band (Mouthful I know), maybe as a Calvary replacement that's weaker but increases great people generation when garrisoned in a city

I only thought about the Buckaroo band because I personally don't like Unique national wonders, but if you don't like that idea, I'd go with a Crazy Horse Memorial NW

Midwest - Happy to hear your SD ideas!

Why did you pick the Maine Class for the UU? Although it was named after Maine, none of them were manufactured there. Why not something like the Porpoise class submarine, which was commonly manufactured in Maine, or the 20th Maine, if you wanted something iconic?

Also, if you want ideas for a South Dakota mod, I have some