Just to throw out some stuff that hasnt been suggested.
Various stage skins? Maybe just pallette swaps to make buildings or objects other colors, change time of day/weather conditions, or even the backdrop. Nothing that would affect the way the stage is played, though. I just enjoy the dusk or night setting some of the Splatfest stages have and I think it would keep things FRESH for map rotation if the stages looked a little different every time, even if random.
Pay more coins to re-roll a single ability slot, rather than all 3. Nothing is stopping people from save scrubbing to get their ideal loadout anyway. Streamlining the process by giving you the option to finetune your gear at a fair price without sacrificing slots you already attained seems reasonable.
Create a new Inkling on a single save? Maybe there is a technical reason why they cant do this? Idk
I also wouldnt mind a gear modifier, at least with the ability to change the colors of your clothes to coordinate it better, but ideally with logo design features that the gamepad would be perfect for. I get that that sort of weakens the purpose of having so many clothing options to begin with, but sometimes my outfit is just so wacky and I dont like using it even though it has the abilities that I want.