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

I'm taking you down like its 1998/1999!

Silver (With A Counter move with a cost of Power) and Metal Sonic

(Advance 1) Angel Island Zone.

Originally for Infinite Battle Now retooled

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

Sumin didnt just make some characters But also sandbag (Though bare bones)

That Hover Move.

It's like the OG game but less stiff.

Giving the character's Icons some polish.

(Will be in the next update)

The "Big Green Man" (Guest) teaser.

(coming some time soon or something.) The 1st Sonic Battle Redux arcade Run: Sonic.