I'd love to see a park like this built just about anywhere, but from a quick glance building this out on that side of the state is a risky bet. The largest nearby metro is Rapid City, population ~70,000. In a couple hour radius of the site it would be hard to come up with a million people. There's already so much going against new theme park properties even when they are built in populated areas, this is just asking for difficulty.
I know there is some tourism to the area in general with the Mt. Rushmore and Crazy Horse Memorials, but this isn't even that close to them. Plus those things are usually done as a stop for long road trips, not necessarily as destination attractions.
All that said though, these people sound like they know what they are doing with the park itself. Unlike so many other recent new properties they understand what a proper investment should be, they have good backing and industry experience, and they have some really good ideas to make a very interesting and immersive attraction. I just wish they would move it down to the south east corner of the state.