I dunno if they will ever expand Tokyo DisneySea and add new ports cause of the use of the "seven seas" term. (Still dont know where that came from.) But if they ever did do one I had a couple of ideas.
My idea of an 8th port would be devoted to the South Pacific/Great Barrier Reef. A section of the part would be themed after the Eastern Australian Coast where there would be live animals on display. And a theater shaped after the Sydney Opera House. What would be in it? Finding Nemo the musical or something like that. They could add some Finding Nemo rides like Crush's roller coaster or something like that.
An idea i thought of today would be an area themed after the show Lost. Where you walk onto an island and you find the Oceanic plane wreckage. It's a playground basically. Though I dunno what the fan base is like for the show in Japan. There would also be a store called "Sawyer's Secret Stash" (no porno or cigarrettes will be sold though) and a food stand designed after "the kitchen" on the show. For the adults and kids there also a exploration base where you walk into an underground base and explore and tour all of the different Dharma stations.