I honestly expected Shifu to die in KFP3, However I thought it would happen since Po was becoming the teacher and since Stoick died in How To Train Your Dragon 2. I also think it would've gave Po mature character development and a decent set up for what we could see of Po in the 4th and final film. I honestly hope they do kill him off in the next movie, now look I love Shifu, he's my 3rd favourite character in the franchise but he's getting old and his time has come. I also think it'd give Po good character independence if they kill him off in the 1st act of the movie to give Kung Fu Panda 4 the feeling of HTTYD 3: The Hidden World the way Hiccup was independent. I also think we should see Po become independent.
I think it would give the 4th and final movie a darker tone like the 2nd film. A mixture of light and dark. Same tone as HTTYD 3 but a little darker.