Dunno if it's been said yet but...
Vader return = Clone.
Seriously, Episode 2 was "Attack of the Clones" and all about them.
Don't you think, over the 20 years or so between "Sith" and "Hope", that Palpatine might have planned for an eventuality such as his own demise and of his protege?
I'm likely to be in the minority for many new Star Wars things (I like that Disney bought it, because it gets it away from Lucas who made horrible prequels) and also, as a Vader fan... I'd love to see more of him!
That said, we could have more Vader w/out him coming back in Episode VII. They could adapt the Force Unleash story-line or some other in-between episodes III and IV story ideas to return Vader to us.