I think with the Inklings holding neutral B should be them continuously shooting the splattershot in the direction they're currently facing, with back and forwards moving them and up and down letting them aim. The splattershot has too high rate of fire to work as a Megaman type jab and the ink bullets should cause no-knockback damage but push opponents slightly away.
I'd like to see side smash as splat charger, where it's incredibly weak at the start of charge but the longer you hold it the further the hitbox goes and the stronger it is, just like chargers in the game.
Dash attack can be a roller swing, and holding A can make you walk and paint the ground with a weak hitbox on the roller.
Jab as inkbrush swings. Just two swings back and forth, pretty simple.
Up and down air can be blaster which has great range but blind spots close to the Inkling and rather heavy endlag.
Side special is splat bomb like everyone else has mentioned. works the same as splattershot controls where you can move back and forth and aim up and down by holding B.
Up special is squid jump of course. A high reaching but no hitbox recovery with slight landing lag like Rosalina's.
Down smash could be ink mine.
Down special could be Bubbler, a special that charges the more you fight and allows temporary projectile invulnerability, nullifying all projectiles that hit you. Just like in Splatoon your head glows whenever it's available to use.
Crouch is definitely squid form. Inkling's run should be one of the worst in the game with crawling no better, until there's ink on the ground which drastically boosts crawl's speed and lowers their hitbox too.
Down tilt could be a quick Kraken spin.
I'd like to see FS being a combination of Killer Wail and Inkstrike. a cursor appears on the screen that you can control, and when you press B again your Inkling will bring out a Killer Wail and aim it at the cursor as an Inkstrike shoots directly downwards on the cursor position too, meaning opponents have to watch out for both hitboxes.
I'm not sure for the other attacks i missed out on, but finding weapons to fit into the gaps isn't hard at all.
Naturally taunts should include the "stay fresh~" pose and the "stay off the hook" pose. Win animations should be pulled directly from the game with the splattershot, charger, and roller win anims.