I don't love the cars idea. But sorta related to this, I always found it weird how "frontier" seemed to be pigeon-holed into the wild west only for some people.
When, like, tom sawyer and the river boat is Missouri. Why does Georgia(now Louisiana), yellowstone, Arizona, Missouri, smokey mountains/southern states(pecos bill and kinda country bears) not all fall under "frontier" for people.
Heck, i could argue the thing that's always been out of place is big thunder mountain given it's the only thing so firmly set in that old west dessert setting.
Like, Adventureland is literally just used as a blanket term for "exotic" destinations. It's all over the place geographically
Im not saying this cars thing will be good, but it doesn't seem out of line on its face to me outside of time period concerns. But MK has time period issues in other places already, as other pisters have pointed out