Frontierland was never meant to be only Western themed. It’s called “Frontier”land – meaning a tribute to the American frontier – the rugged lands that stretched across the entire nation – from the Appalachians to the Great Plains to the Rockies to the Desert Southwest. It’s an ode to the pioneering spirit – American exploration.
Just think of Davy Crockett. He’s Disney’s very embodiment of “frontier,” and yet he only goes West when he fights at the Alamo. At the beginning of the movie, he’s in Tennessee (“greenest state in the land of the free”). In the second movie, he’s moving along the Ohio River with Mike Fink – a place far from the West.
That’s why it’s called the “Rivers of America” in Frontierland, and not “Rivers of the West.” They used to have the Mike Fink keelboats on it. The Rivers of America encircle Tom Sawyer’s Island. Tom Sawyer, like Crockett, was all about exploring.
If you read any of the old official WDW books (that were published long before Splash was even conceived), you’ll see that Frontierland is themed after the pioneering spirit and the timeframe of the 1770s-1880s (roughly the period right after Liberty Square and before Main Street).
The Country Bears fit right in. Even though it’s set in the year it opened (1971), it’s an ode to that timeframe and the ideas of pioneers. That’s why you have Henry singing about Davy Crockett, Big Al in a cowboy hat singing a song set in the old West (Blood on the Saddle), and Teddi Barra being like an old-time saloon girl, and the Five Bear Rugs being like hillbillies. Buff is a buffalo – a creature of the Great Plains. Melvin is a moose – an animal of the north. The entire show takes place in Grizzly Hall – grizzly bears live up near the north. The Bears were never meant to be tethered to one geographic location.
Splash Mountain fits into that theme of the 1770s-1880s, and pioneers, too. Song of the South takes place after the Civil War ended (late 1860s about). The whole story of Splash is Brer Rabbit “moving along, looking for adventure” much like a pioneer. The animated segments in Song of the South (which the ride is based on) are set out in the wilderness – the frontier.
The problem with trying to shoehorn Princess and the Frog into Frontierland is that it fits neither that time period nor does it have a pioneering spirit. Princess and the Frog takes place in 1926. For reference, that’s only two years before Steamboat Willie came out – a far cry from the 1770s-1880s. And, the movie doesn’t have anything to do with being a pioneer. Tiana’s main goal is to open a restaurant in New Orleans (a place already crowded with restaurants). There’s nothing pioneering about that. I'd put Princess and the Frog in Fantasyland. They could use the building that once housed Snow White's Scary Adventure.