It's a new IP and they clearly have a wide range of attacks. I think Nintendo would be crazy NOT to include Inklings in the next Smash Bros.
Smash has a pretty bad track record of putting in characters from games that are just coming out. Like, Melee didn't include Pikmin or Olimar in any way even though Pikmin came out around the same time as Melee. Also not much acknowledgement of Luigi's Mansion (Luigi's Mansion did eventually get a stage in Brawl). Melee did, however, include Roy, whose Fire Emblem hadn't quite come out yet, I think? However, other than that, it's hard to say, because a lot of the Brawl characters were just random stuff from series that had already been around for a while.
I really doubt anything from Splatoon, or Project S.T.E.A.M. or whatever will make its way into the new Smash Bros. I doubt it will be an assist trophy either. At best, maybe there will be a static, regular-ass trophy related to those games.
It would be kinda hilarious if they got some more Capcom stuff, like a Monster Hunter inclusion of some sort or something. I actually find that more likely, though they probably would've announced that by now if it were happening.
I heard somewhere that all the characters that would be in the game were decided on at the start of the project. I don't think that Splatoon could have been far along enough at that point where they would have decided on the look of the characters, or maybe it didn't even exist on a conceptual level yet.