I think there could be a lot of little optimizations, and a few new things to spice up gameplay and that would be enough. It's all things that could be included in Splatoon in regular updates though so it'd be more about the quantity of these updates rather than anything specifically but here are things I would think they would throw into a sequel:
Allow multiple saved Gear/Weapon loadouts to switch in-between matches.
A "For Fun" lobby for the ranked modes.
Completely new weapon styles with their own specific sub/specials. e.g a Hose weapon that instead of having a sub-weapon, the sub allows you to change between styles of shooting with the hose such as short range spray (like if you had your thumb over the nozzle of a hose) or long-range stream. Would function more like a beam weapon. The special could be a giant inkzooka like stream that lasts a few seconds except it's horizontal so it makes a huge wall of ink.
Actually online hub world where you can interact with other players, form temporary squads without needing to add to friendlist, trade/sell versions of gear etc.
Expanded solo campaign which features actual multiplayer modes against AI among the unique challenges from the multiplayer like invisible walls and random enemies.
And of course a slew of new maps, modes, gear and weapon loadouts. Some new types of subs and specials wouldn't go amiss either but honestly the balance they have right now is great and I think they are nailing it.