Adventureland is like calling something Fantasyland or Movieland, it is more of a genre than a region or place in time.
American Frontier on the otherhand took place during 1607 – 1920
"The American frontier includes the geography, history, folklore, and cultural expression of life in the forward wave of American expansion that began with English colonial settlements in the early 17th century and ended with the admission of the last remaining western territories as states in 1912."
Splash Mountain wasn't a perfect pairing with it, but it was much closer than Princess and the Frog. At least the setting feels more like the country than Tiana's, which consists of a bustling city (New Orleans in 1926) and a swamp.
Also the exterior of the new attraction will stick out like a sore thumb. The current resident Splash Mountain blends in better with Big Thunder due to their similar color palettes, which is inspired by dry western settings.
One small problem: The parts of Frontierland that deal with "The South" aren't from the 1600's when "The South" was "The Frontier", but many generations later after the Indians were pushed out and large towns established and territories converted into States. If you're not going to account for the chronology of the Western Expansion, then we can then still call The South as it exists today as still being frontier territory.
Whatever Song of the South is, the last thing anyone would call it is The Frontier or in the genre of frontier stories.
