Everyone can Admit Darth Vader is the most bad ass Sith in the galaxy and his own Anthology Movie set right after Episode 3: Revenge of the Sith would be amazing. Yes, most of his story has been told from his Slavery with Watto to eventually find himself back to the Light Side with the help of Luke and Company but we don't see him in his prime of Darth Vader except for a little in Rebels. With the recent success from Rogue One last night we realised that people still have an interest in the events between Episode 3-4 so this would be a real success for Disney/Lucasfilm but we would need it to be a Gritty and dark as Rogue One was.

I know what you're thinking "But there are already books and games that cover this part of his story, " Well, whether you agree with it or not, these stories are no longer considered canon. 

I'm sure Lucasfilm already has their hands full with many other Star Wars projects, including episodes VIII and IX that we are all looking forward to. but I really believe a Feature Film centred around this story would not only help tie the films together but also be successful at the box office like Rogue One. Maybe even more.

The movie can have
1. Death Star Prototypes
2. Young Tarkin (So he doesn't need to be CGI)
3. Transition from Clones to Recruits and killing the Kaminoans that didn't agree with the empires opinion
4. The most import part we will get to see Vader killing off a lot of Jedi that survived Order 66

(ROGUE ONE SPOILER)
Maybe we can include Galen Erso and his involvement with the Death Star
(END SPOILER)

So Please Lucasfilms and Disney, make a stand alone Darth Vader Anthology film between the event of Episode III and IV!!