Rumor du jour: they're bringing back Darth Vader now
That wouldn't shock me at all. Disney sees Star Wars as a brand that encompasses important sub-brands - Luke Skywalker etc. The other OT characters are also brands, but Vader is the biggest brand of them all.
Rather than trying to bring back Vader in the post-ROTJ trilogy, can't they just do a trilogy set between ROTS and ANH, with Vader as the man character? They have big brands in Star Wars, Luke Skywalker, Han Solo and Leia Organa for the next trilogy, do they really need Vader too?
They could have Luke go all darkside and become Vader, who is not a person but rather a deathless force of evil or something like that. I'd prefer Luke to become Darth Anythingelse, but this is Disney and you can't have Snow White without the Evil Queen, after all.