So I see we're talking a lot about characters, but we don't usually get into the movesets. I'd like to take a shot at Inklings.
I'm fairly certain they're going to be a Stage Control character, using either a neutral A or Neutral B attack to shoot ink around the stage. If an opponent gets caught in the ink, they'll be slowed down and unable to escape necessarily. The Inklings, whose abilities allow them to traverse the ink, will be able to swim in it, and start or continue a combo through this.
Other moves will include the Ink grenades as a Side B or Neutral B, the Mega Jump as the Up B. I'm thinking it might be an interesting idea if the Down B is a transformation into the squid form that would allow you to either reload the ink you've used, and swim in the Ink around the stage.
Another interesting idea I was thinking about. In Splatoon, if you shoot your opponent enough, they are KO'd. What if, in Smash, when you cover an opponent, you gradually slow them down? If you completely cover them, it causes knockback. That knockback could be used to continue a combo or create space between the players.
For the final smash, I think you could see the rocket from Splatoon 1, the Ground Pound from Splatoon 2, the Rainmaker, or something else I'm not thinking of.
Either way, I'm really looking forward to the Inklings moveset. There's no way it doesn't involve spreading Ink around as much as possible and controlling where your opponent goes.