That Hover Move.

It's like the OG game but less stiff.

Despite being Water (Or Mutated Chao) slime sounds

2/3 of them are references to

Sonic's Concept and is also a Skin in Sonic Superstars Cream's Debut (Though inconsistent looks)

A former 90s rival's (A red capped plumber guy) recent 3D platformer since 2017

The "Big Green Man" (Guest) teaser.

Another mechanic that I coded for the first time

"Grab Canceling"

The only thing that varies between character is: how long will they actually throw the opponent while Pressing Y + Holding FWD/Pressing Z + Holding Backwards will still do the cancel trick

surprising how i manage to draw King K. Rool accurately like that for the first time

The Classic Hop and Dive. (Amy Rose)

Yoshi and Birdo cosplaying as Ichigo Momomiya

Act Music as Rounds? (may not apply to every stage)

A returning move from Infinite Battle

but a bit more on Sonic 06's Green Gem.