If we're altering bosses, I have a suggestion about Galacnik. Just a warning though: it removes the gore at the end :) , so if it's necessary to keep it, then my idea does not work.
The overall concept is: being hit by Sonic enough times makes Galacnik gradually lose his powers, making him smaller, but also more mobile. The final two phases are more cutscenes than actual fights.
The first phase is the current GGZ battle, but with a reduced number of hits.
After hitting Galacnik enough times, he becomes visibly smaller, though tall enough to fully cover the left or right edge of the screen. At this point, the camera locks in one place and rocks stop spawning. Galacnik alternates between the left and the right side of the screen (visually he either slides in and out or teleports) and continues shooting energy balls (which are also smaller) and lasers.
Galacnik has shrunk even more (how much? Depends on what works gameplay-wise). He has now become very mobile, flying all over the screen and actively avoiding Sonic, and while he does not have lasers anymore, he can fire energy balls like a machine gun. From time to time, he stops to rest for a couple of seconds, allowing Sonic to hit him. This phase is the hardest.
The music has become quieter. Galacnik has now almost shrunk to Robotnik's size and several elements from his Galacnik design have disappeared. He simply flies in a lazy 8, shooting single energy balls from time to time, but it is clear he is almost de-powered.
The music has faded out. Galacnik has completely reverted to Robotnik and is now simply hanging in the void with his eyes half-closed, immobile and powerless. Touching him once kills him. Visually, he either becomes grayscale and shatters like in the AAZ1 Past cutscene or (I prefer this option) is sent flying back as if he was KOd in Street Fighter with his palette progressively changing to dark blue, then to transparent, so he basically fades into the darkness of space and dissolves.
If we're altering bosses, I have a suggestion about Galacnik. Just a warning though: it removes the gore at the end :) , so if it's necessary to keep it, then my idea does not work.
The overall concept is: being hit by Sonic enough times makes Galacnik gradually lose his powers, making him smaller, but also more mobile. The final two phases are more cutscenes than actual fights.