I've provide countless ideas in prior threads about what I'd like to see added/changed/removed, so here's some of the lesser common ones that don't get thrown around that I'd suggest (which include, for the most part, just aesthetic additions), aside from more content/modes, configuring the appearances of Inkling's appearances, and stuff regarding Octo Valley.
1) In Multiplayer, make the loser theme a bit more comedic and less dramatic/sappy/mocking, something like Team Fortress 2's. Additionally, change Judd's expression to a more scared expression if you do lose, with a gaping mouth and sad eyes, along with the dropping the flag move.
2) Incorporate some kind of feature for the cafe located underneath the Squid Sisters' broadcast room.
3) Low Priority, but having an announcer that speaks the player's set language (not Inkling!!) would be pretty fun.
4) Display visual percentages of area covered as a bar below the eight squids representing each team, like in that Splatoon for Minecraft map.
5) Have the Inklings on the top screen change expressions depending on their current condition (with yours in a colored outline), something like in Doom and Doom II. If they're being attacked, the corresponding squid would appear to be emptying itself on ink, so the upper "black" half would get bigger as an Inkling runs lower on health, until it finally dies. When they're ready to deploy a special, the squid begins to glow and boil like in the game itself, but this only happens with the squids on your side.