4vs4vs4vs4! That would be crazy! :)

A bigger single player is a must imo. Splatoon is an awesome game, but the single player kinda left a lot to be desired ( minus the final boss, which was simply amazing).

This thread has made me realise how many places they can take the concept of Splatoon. Splat vehicles could definitely be fun (although not on every map, or they'd probably get a little annoying). I'm always a fan of team vs AI as in Mann vs Machine and CoD zombie mode. And having turf wars with more than just 2 teams would be chaos - with stunning visuals of 3 colours overlaying each other.

I hope that Nintendo take at least one major game-changing idea such as the ones above and gives it a go for that game. An incremental upgrade would be a shame in my opinion when there's such potential in the series.

They probably add in proper on the fly weapon and gear load outs. I'd also like a character squid creator. And a friends custom map mode. Team color selection.

PS4 or PC support?

In addition to everyone's fantastic suggestions (these are all great ideas), I think some new game modes might not be a bad idea...
A cooperative mode would be cool, maybe eight or more players teaming up to bring down the Octolings...
For competitive modes, maybe a "Sunshine Mode," where one teams tries to make a mess and the other is tasked with cleaning it; they could use mechanics similar to Capture the Flag where the team making a mess is trying to submerge shine sprites in gunk and the other team is trying to keep them clean...
And a mode that combines Splatoon with Mario Paint; I don't know how that would work, but basically I want to see people get creative with projectile ink...

Voice chat more singleplayer, maps, weapons.

I would imagine it would be on the NX home console and here are some features I would imagine it having-

*being able to swim on ceilings, around wall corners, basically more dynamic squid mechanics
*a bigger, more interactive hub. i would imagine since in Splatoon lore, inklings are now the dominant species, so lets have more cities take inspiration. you should also be able to move around the city instead of a cramped area and be able to use weapons in the hub.
*new weapon types, obviously
*the campaign mode would be able to have plenty of new designs but the actual story is a mystery to anyone. maybe it would be left out entirely and replaced with more character dialog and interactions between level ups
*call it spla-two-n
*more gear types. we already have hats, shirts, and shoes. there is potential for pants and who knows what else.
*have more options to get Sea Snails from and more uses for them.
*4 player local spitscreen. either fight eachother, or online as a team

I'd love a ultimate move. A Nuke ink bomb.  A special drop item. Or a new game mode. You break through defensive walls. Think like Payload on TF2. Once you reach the Nuke. everyone fires their weapons. Once the shield fails, it goes off. Potentionally destroying a wall, for access. Or clearing the map for the team.

The number 2 in its name. Spla2oon! It's better than Smash Bros. for Wii U at least.