IMO, all of Liberty Square and Frontierland needs an overhaul. The architecture can be tweaked, and the theme modified to something that allows for logical expansion.
Liberty Square to Frontierland has a complexity of theming that no other land at WDW really has. As you move through the land you are seeing the passage of time and geographic location. Any modification to the area dilutes that theme. They did it with Splash Mountain initially, but the bandaid needs to be ripped off. I'm dancing around an actual solution here, saving it for a podcast discussion but I'm comfortable identifying the problems.
To be clear, I don't think the Frontierland/Liberty Square theming issue is a bigger issue than everything that's wrong with Tomorrowland, but it is absolutely something that will need to be addressed in time.
Tom Sawyer Island and the Rivers of America represent a significant amount of real estate in the park that is underutilized. Something to think about, Haunted Mansion is closer to Big Thunder Mountain Railroad as the [raven] flies than it is to Splash Mountain. Filling in part of the Rivers of America and connecting Haunted Mansion and Big Thunder Mountain Railroad presents a wide range of possibilities. From a theming standpoint, The Haunted Mansion and its variants exist in Liberty Square, New Orleans Square, Frontierland, Fantasyland and Mystic Point when looking around the world.