Such a move would actually make the most sense logically speaking. Not as the primary antagonist necessarily, and honestly I don't know why some of you jumped right to that conclusion. Think more along the lines of a force that sets in motion a chain of events that culminate long after his death. His death doesn't necessarily mean that some long range plan hadn't been set in motion years before his death.
Which ironically enough would really redeem Vader as a villain. Think about it for a moment was Vader really all that impressive. Technically speaking he should have been conniving long and hard trying to find a way not just to unseat the Emperor, but seize control of his holdings. No matter what the old manipulator had cooked up to stop him even after he was dead. Which is really hard, because the guy has the force, a massive fleet, and has a real desire to build a number of planet obliterating weapons.
Turn Vader into a long term schemer like the Emperor, and it actually completes a logical train of thought. There is a rule laid out in the movies, and the majority of the audience isn't going to be thinking of some novels set in a expanded universe when it comes to this. There are only two Sith in the galaxy. If there were two, and then there were none. Then how can there be a new villain, or a supporting cast. Especially when the Empire has fallen. You as a writer would need to explain where the new Sith came from. A legacy of Vader is the only logical choice.
All you need to do is have Vader mustering a secret force outside of the Empires space lead by his very own very secret apprentice set to launch a surprise invasion at a preset time decades later. Granted it all seems a little bit cliche, but I just thought that one up in a few minutes. Another possible scenario that might work just as well is to expand the beginning of the prequels.
I really don't like the whole origin story for Anakin Skywalker. It has never sat right with me given who he actually becomes all of half way through the story. The whole bit about prophecies, and a blatant ripoff of just about all the demigod myths. Well it just seemed really silly even for Star Wars. Given what he becomes half way through the story. He comes across more as a Trojan Horse then a Messiah figure. Which would really up the ante for the convoluted reasoning already at play in his story.
I mean we already have a allusion to a Sith creating life in the Prequels. How beautiful would it be for a unknown Sith to have been going around cooking up children that would explode in the hands of anyone who played with them. Hell you could even have that Sith being the one that created the prophecy in the first place.
Anyway the point is this. Vader making a reappearance wouldn't necessarily be a detriment to the next movie, or to the franchise. There are a lot of ingenious ways he could be used, and frankly it really helps with the whole continuity.
