Ascended Fanon: Fans were clamoring for playable Octolings, with many believing that new single-player campaign with them as the protagonists would be a great way into introduce them. The developers would have a friendly Octoling in the form of Off the Hook's Marina at launch, and later deliver on the race as playable characters with the Octo Expansion DLC campaign.
Off the Hook technically became this, as Rina Itou and Alice (the voice actresses for Pearl and Marina) ended up hitting it off quite well after working on the game, and the two of them would continue to do their own music collaborations with Itou's band Lighter190E.
The Milky Eminence gear set was based on a previously-fictional team uniform used by Emperor's gang in the second CoroCoro manga.
When the game went live on the Nintendo Switch eShop in North America, users were unable to access it, repeatedly receiving error codes.
Once Japan got word that the Milky Eminence outfit could only be obtained through download codes in CoroCoro Comic, magazines went flying off the shelves.
While the eShop itself survived thanks to preorders, the release of the Octo Expansion DLC brought Nintendo's distribution servers to their knees, resulting in many people taking hours to download the content, even on very fast connections.
Splatwon (or variations such as Spla2n and Spla2oon) spread like wildfire among the English-speaking community from the moment the original game became a hit, to the point where some fans were disappointed that the pun-loving Nintendo didn't utilize this title. (Spla2n could be clever, but Splatwon could just as easily be read as Splat-Twon. Plus, the pun would only work in English anyway.)
The Splatfest-exclusive stage Shifty Station has been dubbed by several fans as the "Twilight Zone" due to its Japanese name "Mystery Zone" being the same as the show's Japanese title.
To differentiate playable Octolings from their enemy counterparts, which are both officially called "Octolings" in the Western versions, some Western fans call the enemy versions "Octozons", which comes from their official Japanese name, Takozonesu (tako = octopus, amazones = amazons), while the playable versions are called Tako.
The Bloblobber is often referred to as the "Toilet" or "Kitchen Sink" due to its appearance.
The fandom was really quick to notice that the "Booyah Bomb" special is a Splatoon-ized Spirit Bomb, and commonly refer to it as such.
The Clear Dapple Dualies are often referred to as the "Clapples".
After the Hero Mode trailer dropped, many people have predicted that Callie was brainwashed by DJ Octavio. Many people also expected that the final boss theme is a Dark Reprise version of "Bomb Rush Blush" (Callie's solo song), and that Marie would pull an "I Know You're in There Somewhere" Fight via a sad version of "Tide Goes Out" (her own solo song).
Soon after the game released, pointers were discovered in the data for "OctGirl" and "OctBoy" as options for character customization, but with nothing associated with them, leading to the "playable Octoling" rumors from the first game to resurface. An early balance patch added more fuel to the fire, with dataminers discovering character attributes for the player's species in addition to the regular customization options, like gender, skin tone, and hairstyle, alongside unused model meshes that looked suspiciously like hair models for both genders of octos, separate from what the NPCs use. Playable Octolings would be added to the game with Octo Expansion DLC campaign, released a year after the game's launch.
After the announcement of the Octo Expansion and the fact that it's located in a subway, many keen eyed fans noted the inaccessible subway entrance in the main plaza, down an otherwise dead-end path. While one could argue the path was merely there for symmetry with the entrance to Salmon Run, these fans speculated it would be the entrance to the Expansion's content and had been hiding in plain sight since day one. Even the sprite artwork of a Gunion (an octopus-like enemy from Super Mario Land) seen in the subway entrance made it more obvious. They were correct.
For the Final Splatfest, many players speculated that "Fly Octo Fly ~ Ebb & Flow", the Final Boss music from Octo Expansion, would be used as Turf War music. This was confirmed on the last day of the Splatfest, where it played for all matches.
Just like in the first game, fans predicted that the results of the final Splatfest would affect the plot of the next game. In Splatoon 3, the Inklings/Octolings descend into "the wake of chaos" in a new territory.
The "Squid vs. Octopus" Splatfest took place on July 21, 2018, the anniversary of the game's international release date. It qualifies both in-universe as well, where it's referred to as the first anniversary of Pearl and Marina hosting the Inkopolis News program.
The game's final Splatfest, "Chaos vs. Order", took place from July 18 to July 21, 2019, the weekend of the game's second anniversary.
No Export for You: Strangely enough, this applies not to the game itself but to certain lines of equipment: the Milky Eminence gear (based on the original gear worn by Emperor and his crew in the Splatoonmanga) the Spy gear, (based on suits and ties, like stereotypical spies), and the Mecha gear (a cyborg look designed by a fan of the franchise), which were only available through a limited number of download codes in CoroCoro Comics magazines—which have never been, and likely never will be, available outside of Japan. Obnoxiously, even if you do run into a Japanese player wearing a piece from these sets in the Square, Murch won't let you order them. Even more obnoxiously, the manga is also available internationally, so remedying this would theoretically be easy as pie, but neither Nintendo nor Viz (who localizes the manga) have expressed any interest in doing so.
Prop Recycling: The assets used for holiday themed Splatfests after Splatoween have been reused. More subtle with Frosty Fest, which takes the ectoplasm used for Splatoween and colors it white to represent snow, but more obvious with Spring Fest, which just makes the same prop grass green. The latter also has Easter egg recolored versions of Splatoween's skeletal spider crabs decorating stages.
Refitted for Sequel: Or DLC. Many of the stage ideas and concepts used in Octo Expansion were initially planned for the base game's Hero Mode.
Had Team Callie won the final Splatfest back in the first game, Marie would have been the one to go missing instead.
At one point, Off the Hook was going to be an Octoling duo. Even after it was decided to make them mixed-race, Pearl was to be the same height as Marina and similarly wear a midriff-bearing outfit.
The Testfire versions of "Inkoming!", "Rip Entry", "Undertow", "Now or Never" (Wet Floor) and "Turf Master" were slightly different from the final release.
Octolings were briefly planned to be available as playable characters at launch in the base game, but the developers decided against it, feeling that it wouldn't make sense to do so without having some in-universe justification for why the two enemy races were suddenly hanging out with each other. Octo Expansion was developed as a result.
Also concerning Octolings, the original art◊ and model◊ for the Octolings back in Splatoon 1 featured purple eye lining, as opposed to the black domino-mask-esque markings of the Inklings; you don't get to see this in the game as published, but the art did show it off. The eye lining also markedly tapered away around their eyelids, eventually vanishing almost entirely. The eye lining also has a very notable "point" which angles toward the cheeks on each eye. The final playable Octolings in OE, however, had some changes: while the general concept of the eye lining was kept, the color was shifted over to black in order to maintain better visual continuity with the Inkling player characters, and while the basic idea of the tapering at the eyelids was retained, the eye lining is still very visible on the upper parts of their eyes. The "point" was also retained, but was made significantly wider and more visually distinct, and more importantly, it became a visual feature exclusive to the girls; the octo-boys simply have thicker lining around their lower eyes.
In the early build of Octo Expansion seen in the Direct reveal, the subway lines were delineated by number rather than letter, with CQ welcoming Eight to "the 3 line" in what would be the C Line in the release version.
Similar to the first game, dataminers regularly uncovered the alternate Off the Hook dialogue that would have been used if the other team had won for every American/European Splatfest from "Mikey vs. Donnie" onwards, and every Japanese Splatfest from "Hello Kitty vs. My Melody" onwards.
The Splatoon 2 artbook shows original drafts of Callie's plotline, including her being Not Brainwashed and morosely living among the Octarians, with her getting her own mech and the final boss fight including backup dancers and a different Octobot King mech.