From the highest heights of the Matterhorn to the deepest depths of the ocean, the Northeast quadrant of Disneyland has always been a place of the remote and the mysterious. Guests have been able to explore these regions and discover everything from sea serpents to abominable snowmen to the lost continent of Atlantis. So, the obvious solution to the problem of the Northeast quadrant having no discernible theme is to create a fifth realm: Discoveryland.
The Matterhorn never fully belonged to Fantasyland, and the Submarine Voyage never fully belonged to Tomorrowland. But, in Discoveryland, these two attractions can fully embrace the core themes both share. And, an entire community of explorers can find a home here amongst the mountains and seas of the world.
Imagine that the passages on either side of the Matterhorn depict Andean and Himalayan settings while the North side that connects to Fantasyland remains Alpine. Imagine also a new and fantastical submarine voyage sans Pixar.
The former Fantasyland Autopia and Motor Boat Cruise have real estate available for the development of new attractions and other features. The monorail beam and PeopleMover guideway can be concealed. The main entrance from Central Plaza can appear more mountainous. And, there can even be multiple levels of walkways that might make better use of the vertical space in the area.
The portal between Fantasyland and Discoveryland might, itself, have multiple levels. For example, Wonderland might be accessible by a rabbit hole, whereas the parade route remains on level ground. Crevasses might appear here and there, while the glaciers that formed the Matterhorn's hook and glacial pool may also be depicted along with some volcanic activity for interesting juxtapositions between fire and ice. And, the portal between Discoveryland and Tomorrowland is the perfect place for Tony Baxter's favorite dirigible, the Hyperion, to be anchored in order to form an appropriate transition and a visual barrier.
Think "Around the World in 80 Days", "In Search of the Castaways", and "The Island at the Top of the World". Literature is full of stories about exploration and discoveries in remote and mysterious places. So, to Frontierland and Tomorrowland and Adventureland and Fantasyland, I advocate the addition of Discoveryland as the fifth and final realm.