I'll share both the kaiju and Sonic character goal themes, I think you'll like my choices

Kaiju goal music is the Super X theme, and Sonic character goal is the military march, both coming from Godzilla 1984.

King ghidorah as Eggman?

nah...I do have plans for Mephilis to posess King Ghidorah, then Dark Gaia. I just need boss, stage, and music choices for said stages

For the King Ghidorah Boss music, it's King Ghidorah (1991) theme (I guess)

I guess then we could do a sort of mashup between the 2019, 1991 themesand Endless Possibilities. But that's the final boss...any other monsters then Hedorah, Mechagodzilla, and Rodan controlled by Eggman? I need some Eggman bosses, normal stage ideas, and music choices for those. But then again, the Sonic Unleashed day boss theme should do for Eggman bosses. And for Rodan and some other monsters controlled by Eggman, like Kong, Vs. Character from Sonic 06 should do.

Ok, what theme for Godzilla?

He isn't a boss
But I guess the Monsterverse themes since this is the Monsterverse

Not as a boss, as a hero.

ooh! I have an idea for special stage music
Rude Buster

Cool

if you wanna hear it, it's this: https://www.youtube.com/watch?v=Lpw7SzzIfnY

again, all of his Monsterverse themes should do

Also, I'm struggling over which Mechagodzilla I should do: Showa or Kiryu? Eggman would build it, and while Kiryu is a cool opponent, Showa has such a BANGER THEME that would make for a super fun boss.