This rarely ever works for movies. The best thing Star Wars could do is create new villains. Maul was great. Kylo is fine.
Watch the YouTube video Dutchrudder posted and apply that generally to iconic villains.
Iconic villains are great because they're foils to the hero and exist as a mysterious, creeping malevolence always threatening to destroy the good.
Featuring the villain forces them into a protagonist role, which requires a lot of baggage and explaining things about the character, which ruins the mystique.
Besides, there are only a few ways to go with these 'villain as the hero' movies:
1) you can play it for laughs or make it meta. Think later Freddy Krueger and Jason movies, American Psycho, anything from Tarantino
2) you can make it really dark, like Taxi Driver, Devil's Rejects, Hannibal, The Shining, etc. but the problem there is that Vader would need to be a psycho, which he's not
3) you can make Vader a tragic figure, like Frankenstein, Darkman, The Fly, or Maleficent, but this creates a sympathetic villain. Lucas already tried this and failed. It's the hardest to pull off and has never been done with a recurring character.
As for me, I like Vader and Boba Fett and every other villain being mysterious badasses. I don't need or want to know what they eat for breakfast, what underwear they prefer, or why they are the way they are. They exist, and they're awesome. That's enough.