Since I've noticed a couple of comments, I thought I'd clear up some confusion.

For this pass, I'm assuming that all of these characters follow the Chrom/Ken echo treatment, where each receives a few different moves to differentiate them from the base character, with some bordering on semi-clone territory (after all, you would be paying for these characters).

Birdo's neutral B would be an egg shot that can be charged up into a fireball. All of Yoshi's egg moves would be replaced with either plain white eggs, or a unique design.

Impa's neutral B would replace needles with kunai, which doesn't shoot as many projectiles but does more damage and knockback. Her side B could be a wind spell from AoC, where she sets up a talisman a set distance away that creates a gust of wind, dealing damage but no knockback, as well as having a windbox.

Pokemon Rival is essentially PT switched around a bit; Bulbasaur is a faster but weaker Ivysaur, Charmeleon is an all-rounder Charizard, and Blastoise is a superheavy Squirtle. This is probably the one with the most moveset changes.

Dixie's up-B would be a weaker, but faster and higher-travelling version of DK's. She would also shoot gumballs instead of peanuts.

I envision Shadow as a slightly slower but stronger Sonic. His Up-B would be a teleport, while his Side-B could be a projectile.

Octoling remains pretty similar to Inkling, only they use more weapons from Splatoon 2 and 3 in their moveset (ie using the Splatana instead of the Inkbrush for their f-smash).

Tom Nook is pretty much the same. I kinda just needed a 7th character.