Would DLR's Finding Nemo: Submarine Voyage be better if it went to WDW's Submarine Voyage if it didn't get filled up and turned to Pooh's Playful Spot? Then DLR's Submarine show building, Autopia track layout, and Motorboat Cruise, and Matterhorn could be a whole new land. Or keep Matterhorn in Fantasyland (depending on the new land's new theme). Maybe the area east of Small World Mall could house a couple dark rides. Then the rest of the land could be a Discoveryland, Myths and Legends, or whatever is decided.
I think leaving DLR's lagoon using it for an underground ride or whatnot would be more useful than filling in WDW's lagoon.