The Easter Egg where the Squid Sisters wave at you takes a more heartwarming light after the Hero Mode. Namely, they're happily greeting a dear friend who saved the town, the town's power source, and their grandpa.
When you complete a single player mission again, the Zapfish you normally get is replaced by a stuffed toy Zapfish. Agent 3 appropriately treats it like a toy instead of being in his/her Item Get! pose.
The credits suggest the Inklings and Octarians made peace. There's at least two fun/adorable little scenes with an Octoling interacting with the Hero character- one for the male, one for the female. The Male Inkling apparently stole the Octoling's Goggles, and she demands them back. The Female Inkling seems to have taken the Octoling clothes shopping. Octo Expansion of Splatoon 2confirms that while the Octarian government still held a grudge, many Octolings were inspired by the Calamari Inkantation to leave their oppressive society and integrate into Inkling society.
Likewise, returning to Octo Valley after beating singleplayer reveals a few changes have happened. The most major of these is that Capn' Cuttlefish is keeping DJ Octavio in a nice snowglobe to keep an eye on him. A pre-Turf War scroll shows younger versions of the two acting friendly toward one another. All things considered, Cuttlefish could have put Octavio in a lot of worse places than a pretty-looking giant snowglobe, so it's possible that even after 100 years and terrible war, he still cares about his former friend.
Quite a few of the images seen during the credits are adorable, like Callie and Marie dancing with Cap'n Cuttlefish and Agent 3 awkwardly trying to keep up with Agents 1 and 2 while they pose.
On June 26 of 2015, when gay marriage was legalized nationwide in the United States, many Miiverse posts relating to #LoveWins were displayed by players and as graffiti in Inkopolis Plaza. It certainly helped that when a Japanese Splatfest run with the topic "What will save the world, Love or Money?", Love triumphed in that Splatfest as well.
When that Splatfest was re-run in North America in February 2018 via Splatoon 2, Love triumphed yet again!
On July 12 of 2015 it was offially announced that game developer and Nintendo's president, Satoru Iwata, passed away the day prior. The following days saw Inkopolis plaza saturated with Miiverse posts bidding him farewell, and several players set up matches just to hold 8-squid Charger salutes in his memory.
When the Final Boss takes the awesome levels Up to Eleven, the Calamari Inkantation that begins to play gives Cuttlefish the strength to break free from his ropes and he starts dancing vibrantly. Apparently, the Squid Sisters' music is so damn good that even the Big Bad of the game, DJ Octavio, can't resist but dance along to the beat himself, even during the intense climactic final battle against the player. His occasional chuckles heard in time with the song's section changes during the final phase may further confirm this.
The Final Splatfest, which pits the Squid Sisters directly against each other, caused the fandom to respond by creating fanart that ultimately depicts the sisters either calling for players to vote for each other instead, or embracing each other with the message that the Splatfest will have no bad blood left by it. Callie herself even offered to join Team Marie during the North American announcement.
In the results dialogue, the two show that no competition will ever tear apart their friendship, and the Squid Sisters will be together forever.