I have the same problem with this idea that I have with the comics: they won't do anything, perhaps can't do anything, that won't water down Vader. Vader comes with an inherent problem, namely his redemption.
You could have Vader doing delightfully villainous things that make the sacking of the Jedi Temple look like a slow Tuesday. Annihilating foes and crushing all who stand before him. But how does that dovetail into his ultimate fate? Either you undermine his redemption in ROTJ, by having it turn out that the throne room is the only good deed he's done in decades of being a homicidal thug. Which ultimately makes his redemption hollow and without real meaning. You just can't walk back some things, no matter how you try.
Or he has to be an ever conflicted tragic "hero." Who can never be a true monster, and thus can never really be anything more than what we've already seen. I don't need to sit through episodes of Vader being secretly mopey because he hates himself for being the Emperor's lapdog.
The drama of Vader comes not from the character himself, but from his interactions with the people who knew him when he was Anakin (or would have wanted to, such as Luke). And we've covered all of those bases already.
In order for Vader to be effective in any other context, I think he has to be an antagonist to other characters. Characters who we can be made to cheer for, and who we'll want to see escape him, but who we know probably won't. Use the "legend" of Vader, but not the man. The less we see of him ,the better. Because every time we see him, he loses that mystery, and that edge. Vader is effective because of what he MIGHT have done, what he was capable of. Think about the way he's used in the OT prior to the ESB reveal. Less is more.
