Well KP4 was actually originally gonna tackle this, they were gonna make Zhen & The Chameleon humans who came from another world but it was scrapped because it would raise the question on why there were now humans and would take away screen time from Po.
Yeah I was about to mention that. Originally Chameleon was supposed to a human sorceress named Collector. She and Zhen would come from a place called Hu-Man city. I think I also read that she was originally supposed to have a power of travelling from a universe of humans to the KFP universe and this was changed to the sorceress becoming a Chameleon and instead pulling spirits from the spirit realm.
I actually blame Universal more. In the reddit session, it was mentioned that they wanted a never before thought idea would make the movie interesting so Mike came up with the humans idea. They dropped the live action part very soon and continued with animated humans before dropping them out. I mean first drafts of a movie hardly make it to the final movie.
