At last, we have Challenger Pack #7, which will be available next Tuesday (the 13th). Minecraft is being represented in the game now, with Steve, Alex, Zombie, and Enderman all being playable. Three Mii costumes are also based on Minecraft. Still no sign of Octoling, or a Mii costume of one yet. Still no Pearl and Marina Mii costumes yet, either.
Regardless, the most logical choice for the next fighter from the Splatoon series is Octoling, and there really should be a Splatoon 2 stage in Smash. The best I can think of would be MC Princess Diaries, as the Splatoon 2 stage. Pearl and Marina would be background characters, and the NILS statue would be in the background. Like I said in previous posts, song tracks that can be added include Nasty Majesty, Shark Bytes, Fly Octo Fly, Ebb and Flow Octo, and possibly Into the Light. Maybe some of Dedf1sh's songs as well. Four or six of the Octoling variants can be based off Agent 8, while including at least one variant of both the Enemy Octoling, and Sanitized Octoling.
List of Splatoon Mii costumes I'd like to see that haven't been added yet:
-Octoling Boy and Octoling Girl - if not fighter (Gunner)
-Enemy Octoling - if not fighter (Gunner)
-Pearl (Brawler)
-Marina (Most likely Gunner)
-Cap'n Cuttlefish (Swordfighter)