The “Darth Vader hunts down and terminates Jedi” movie sounds cool in concept, but would probably be hard to write in a compelling way. You’d need to establish the characters of Vader’s victims - and you can’t even use Obi-Wan as one of them. And as the third movie in a Trilogy, there’s just not really enough material that ties back into the pre-Vader story lines (stuff from Episode 1 and 2, like the Clone Wars, etc.) that also overlaps with a post-Vader Episode 3 murder-fest story-line. I think a story about Vader hunting down Jedi survivors would potentially be easier to implement as a series instead of a single movie, as it would necessarily require establishing new characters who are Vader’s victims before killing them off.

In my opinion, despite how cool it sounds on paper, watching Vader kill lots of Jedi actually wouldn’t be as interesting as it sounds. The real gold that could have been mined from the Prequels was the relationship/friendship between Anakin and Obi-Wan and Anakin’s heart-wrenching betrayal of his mentor and best friend. But bizarrely, that was barely explored in Lucas’ Prequels.