There will be expansions to let players revisit locations from the first two games.
As interesting as the Veiled Sea sounds, a lot of fans would love to see the Manoa Lai Sea or the various Blue World locales in glorious high definition, so it would make sense.The whereabouts of the other Blue World legendary creatures
With the confirmation Cacao Maharaja is returning in Luminous, it's possible some of the other Blue World legendaries have also migrated to the Veiled Sea. The most likely candidates are the Ancient Mother and Gungnir, as their backstories indicate they have already traveled great distances and different types of underwater conditions before, with the former traveling from the temperate Manoa Lai to the Arctic Sea and the latter traveling from the tropical Gatama Atoll to Ciceros Strait (assuming Jean-Eric's dialogue isn't a developer oversight.)Additionally, Thanatos will indeed be the shark using his Sakana Connection model, but, having been created as Gaia's Vengeance, will have mellowed out from the Veiled Sea's isolation and the story's increased focus on preserving nature.
The game is set inside of a virtual simulation
When the overview trailer discusses fish having "unusual biometric signs", the fish on-screen pixelates as if it's glitching for a second. It would also handily explain the presence of all the prehistoric and fantastical creatures, along with why the player character looks so artificial compared to the first two installments. Plus, remove one letter and you could spell the title as Endless Ocean: LuminOS. New animals, features, and locations will be added with updates and/or DLC
We're missing some obvious picks for creatures (basically the entire Shore Life category, axolotls, frogs, mantis shrimp, benthic octopi, jellyfish, comb jellies, sea angels, etc.) a few features from previous games (the aquarium, petting/feeding animals, going on shore, etc.), and locations (well, just one, a freshwater environment), and some less obvious additions (alien oceans, crossover legendary creatures, etc.) could be fun. We know the game supports DLC, and it'd support updates (which other Nintendo games such as Splatoon have used to add content as a sort of free DLC) either way for bug-fixing purposes.