People have said this before, but I’m really unsure how it would be done. When people bring up the idea of showing more Vader hunting down Jedi, what would that story look like? How would you do it without making it a very long montage? You could pick one or two or a group of Jedi for him to go after, but why should we get attached to them in particular? The major characters up to this point are all accounted for, so you would have to make up some new ones just for them to die by the end of the movie. Obi Wan’s summary of the situation in the original movie pretty much covers it.