I might make a Rodan sprite, since he is EASY to make. If he has the 7 Chaos Emeralds, and is killed in a level, he becomes Fire Rodan, with none of Sonic's rings to waste, since it's infinite time playing as Fire Rodan, which is a pretty neat trick!
For King Ghidorah, if he HAS been beaten a lot of times as a boss, then he becomes Mecha-King Ghidorah at the final boss. As a playable character, he doesn't turn into Mecha-King Ghidorah.
Another transformation is, if Godzilla has all the 7 Chaos Emeralds, he becomes Burning Godzilla if he gets 50 Rings, or 50 G-Cells.
(I might add G-Cells below "RINGS" in the Sonic 3 UI, which will be awesome!)
And there's more, such as SpaceGodzilla being both a boss and a playable character.
Godzilla's victory pose, the "
shē" dance,, also known as the "Jump Dance", as a attack, performed by Godzilla, in the 1965 film,
Invasion of Astro-Monster, will be used in a Sonic 3 mockup. Also, a special sound effect that would either be him roaring, commonly when beating a level, or if he has beaten a level, and at a low random chance, during victory, Godzilla would shout out, "The King of Monsters is just getting fast, and that's me!"