Hot take: General Kai and Lord Shen should have stayed dead
Shen’s story of tragedy in KFP2 had a beautiful beginning, middle, and end. His own weapon crashing down upon him as he closes his eyes to finally accept his fate is the perfect conclusion to his arc. He also is removed from the more mystical/spiritual side of KFP compared to Tai Lung and Kai, being very much rooted in technology. His only real mystical superstition is the Soothsayer’s predictions.
Kai admittedly had a lot of his storyline cut, but his role as an existential threat beyond anything ever seen before is undermined by his return and capture by the Chameleon. It’d be like if Galactus was captured by Kingpin lol.
Tai Lung imo was really the only villain worth returning for KFP4. He served his purpose in 1 but the focus there was more so on Po. There was a great opportunity there with regard to the things left open-ended regarding his and Shifu’s relationship. Of course, they didn’t really end up doing that anyway, but he was the only one in theory who actually made sense to bring back.