Can they really theme a flat ride in a way that doesn't degrade the thematic immersion of the land? I mean, it's a village and a mansion, and a windmill...mostly. If you ignore the netted coaster track, It's amazing placemaking, not much to remind you that "you are just in a theme park" rather than a real-world place, so to speak. But a flat ride, even a themed one, will take it all down a notch, right?
I do believe they should "go vertical" in the remaining space they have, as they simply don't have enough acreage to exploit the Monsters theme to the level of (likely) demand. A two-level show building (say 100 feet tall) could be concealed behind a dramatic rockwork cliff (Cars Land cliffs at DCA are about 125 tall)...a great backdrop to the land/mansion.
The ground level would have a Blue Bayou style indoor always-night "black lagoon" with scientist camp and boat dock (restaurant, bar, lots of storytelling props and maybe actors, like Adventurer's Club). You'd see ride vehicle boats heading off ominously into the swamp. The attraction could be all overhead on a second floor (boats go up an elevator lift disguised themed as a hurricane storm).
Or, if the second floor isn't needed for the boat attraction, put an indoor in-the-dark suspended coaster up there themed after Dracula...kind of an answer to Disney's Monster's Inc. coaster. It would not have huge hills and valleys but be more swooping/flying like Hiccups so it wouldn't need as much height.