Alright, Chun-Li I'm going to throw out of the conversation. I realize there's no way I can justify that. I'm taking the L on that one.
Sora I know doesn't 100% match Cloud due to giant buster sword vs smaller key. I get that. But, I think I have an idea based on Cloud's movements and a few other things. That may blow up in my face though. I'll admit that. I'll explain later.
Eggman I think maybe if we enlarged the model slightly and replace some of the clown car weapons with same attack style but different look, it could work. Most people I've seen suggesting Eggman basically say he's be in one of his egg machines anyway.
The issue is that for a character to be an Echo they basically need to be derived as much as possible from the original model. That's why Isabelle isn't an echo; they couldn't make her without making a new model and skeleton.