The idea of another playable character springs to mind another idea: co-op mode, Robin and Mitch teaming up to lay the smackdown on the bad guys. They'd have to share the Powerpuff Girls, though it would help to emphasise teamwork between players. =D
Nice to hear you like the ideas of Sedusa being a secret boss and that Robin fights her across the three areas. =) I saw the work-in-progress preview of her on your Twitter account and she is big. Judging by the ground (or is that the rooftop of a large building) also in the shot, she towers over even Robin; perfect for a significantly harder secret boss. From the preview you posted, I can picture Sedusa having a variety of attacks: swiping the ground and air with the staff, her snakes striking from above and the sides and her smashing her other fist down; in addition to the beam attacks from the tiara and necklace and the electric attacks from the snake head. That's what I picture, at least. It's your game, after all, and I'm curious as to what you've got planned for it. =)
If Robin wears the artifacts as a giant after defeating Sedusa and causing her to shrink, then Robin's going to be really big to her. Then Sedusa will know how it feels to be loomed over to such a degree. =D
By the way, I discovered that this game came about because it was a commission via the "Bigger Than The Multiverse" tier on your Patreon. How does that work, exactly? It might be neat to help bring more size-themed games into existence, so I'd like to know details on how that process functions to know whether its something I'd like to sign up for or not.