I think theres a good bit that needs cleaning up from 2 but I think the biggest points that need to be fixed for me are the gyro controls and the gear
The gyro as it stands in splatoon 2 with its input lag is dreadful, and as far as im aware there have since been switch games that have managed to eliminate the lag issues, so its just a matter of the splatoon devs caring enough to fix it for splatoon 3. I understand splatoon 2 was probably pretty rushed to be a launch window game for the switch, but if this gets left as is for 3 its going to be seriously disappointing.
As far as the gear goes, I think now is the time for the devs to finally do something interesting with it. Gear was really powerful in splatoon 1 pretty much out of pure accident as far as I can tell, and things like quick respawn and damage just so happened to work as a good balancing mechanism for that game's weapons.
With Splatoon 2, they were so reticent when making gear that the game launched with almost nothing worth running. Over time they ended up having to artificially increase the value of abilities by giving them insane curves for single subs, ie inkres, special saver, qsj, so on. These abilities did not become powerful, they simply became worth running because they required almost zero investment.
In Splatoon 3 I hope that the devs look back and learn from both splatoon 1 and 2, and revamp the gear system to be more specialized than before.
They could accomplish this in a number of ways, but my personal idea is to axe the sub ability slots and have 4 main ability slots (add one for pants or something, idk). From there, they can create and revamp abilities to all be more desirable than before. For example, make inkres work like it did in splatoon 1 v1.0, revert stealth jump to invisible jumps, rework bomb defense to halve bomb damage, combine swim and run speed into a mobility up ability, etc. The point would be to have EVERY ability feel powerful and game changing to a degree, but of course you would be limited to only running your main ability slots now.
And for the hell of it, a far out idea that will never happen but would be sick is a halo style map editor, but yeah im not expecting that to be a thing in splatoon ever