I would like to preface that I'm terrible at stage design, so I am only going to discuss a few of my ideal picks for Octo Expansion stages rather than submit:
Following the scuffle between Agent 3 and an Octoling, the Octoling (along with Captain Cuttlefish) loses their memories. In an attempt to regain their memories, they move onboard the Deepsea Metro and are christened Test Subject 10,008 by a telephone. This telephone promises their escape from the Deepsea Metro, provided 10,008 (called Agent 8 by Captain Cuttlefish) clears a series of tasks and obtains a number of "thangs." Along the way, they are assisted by Pearl and Marina, who lend monetary and technical support to the duo. After a series of tests, Agent 8 is able to collect all 4 thangs and present them to the old telephone. The telephone and the thangs merge into a machine that suspiciously resembles a blender. Agent 8 and Capt. Cuttlefish enter the machine, with it being their promised vessel of escape. Turns out, the machine is actually a blender. The duo is saved by Agent 3 and begins their escape. After a fight with a brainwashed Agent 3, they reach the surface, where they are greeted by several helicopters belonging to Off the Hook. They are rescued, but not before a giant statue rises from the waters.
The telephone, revealing its name to be Commander Tartar, claims that Inkopolis is a worthless cesspool of a city focused too heavily on fast fashion trends and silly turf wars. It intended to use the data of the previous test subjects to start anew after wiping out the city. As it begins its assault, Marina quickly thinks of a plan in order to slow down the statue and thus, save Inkopolis. Hyperbombs will be dropped on the conveniently solar powered statue, which will explode in ink to delay energy intake and thus allow Pearl to wipe it out with a Killer Wail.
As such, I believe the NILS Statue, if chosen as a stage, should focus mostly on its iteration as a final boss. The giant statue head would be periodically covered in Marina's hyperbombs, which could be a potential stage hazard. The statue itself consists of 5 distinct parts: the chest, the back, the neck, the right arm, and the head, all of which receive a few hyperbombs from Marina. Ride rails and trampolines surround the statue, providing Agent 8 with a means of transportation around the statue. In the background, a cloudy sky filled with helicopters provides an atmosphere that one would not expect when thinking of a Splatoon stage. One of these helicopters, the largest, would have Pearl and Marina (in their Expansion outfits) in it, as well as Captain Cuttlefish and an unconscious Agent 3. The lights would change based on the background music.
The NILS Statue is an extremely important location, being the final boss of Splatoon 2's Octo Expansion. It is a story level as well as a way to incorporate a multiplayer mode (Turf War) into a single player mode. As I mentioned before, a vaporwave based stage would be amazing and a stage based on a statue seen much in vaporwave culture would fit perfectly. Finally, the NILS statue also appears outside of Octo Expansion in the final Splatfest stage, that being MC.Princess Diaries, where its remains can be seen floating in the water.
After Agent 3 saves Agent 8 and Captain Cuttlefish, Agent 8 embarks on a series of stages in order to escape from the Deepsea Metro. They use Agent 3's CQ-80 device to activate an elevator to the surface. On the way, Agent 8 must obtain a weapon through stealth and find a power source for the elevator. On the final stage, they are ambushed by a brainwashed Agent 3 in the elevator.
The escape portion of Octo Expansion is a tough segment intended to gauge what the player had learned throughout the many stations of the Deepsea Metro. It is divided into 7 parts, those being the Coccyx, Villi, Belly, Intestinal, Diaphragm, Peristalsis, and Spinal phases. All of these being named after the body, it could be inferred that this is the inside of the NILS Statue.
Unlike the NILS Statue and the Deepsea Metro, the aesthetic of this portion is extremely different - being mechanical and sterile rather than vivid vaporwave. Being a heavily guarded stealth mission and a way to test the player's prowess in the game, it could have many stage hazards such as rotating platforms, lasers, and enemies. I believe that this portion would be a great stage simply because of the feeling of power the player gets - it is like a climactic victory lap with every new phase being more and more powerful.
Finally, there is the Deepsea Metro. A train run by the enigmatic C.Q. Cumber, its patrons are residents of the deep ocean. Its curious characters help Agent 8 regain their memory, as well as serve as a place for Agent 8 bond with their benefactors. Being a tool of the Kamabo Corporation, it serves as a way to escort test subjects to different stations.
A travelling stage, the Deepsea Metro could possibly visit the various stations in the Octo Expansion. It is one of the most visually stunning pieces of the Switch Library, with its calming music and dark backgrounds creating an environment completely different from the loud turbulence of the surface. Many of the items and obstacles are based on retro items, including Nintendo consoles, appliances, and patterns. The stations are named after iconic pieces of media of the era such as songs and films.
The Deepsea Metro is the most recurring place in the Octo Expansion, being the player's hub world. It could feature cameos from characters like Captain Cuttlefish, Iso Padre, and C.Q. Cumber.