It depends on what the story is. If we have an older Jedi Master Luke, then it'd be a little more organic to have Vader in some capacity - ie: the Luke is having an emotional struggle with the pressure of reforming the Jedi Order, which is related in some way to the overall new threat/enemy of the new trilogy. Whenever Luke mediates, he's haunted by visions/memories of Vader - quick and haunting visions of Vader destroying Jedi/hearing the ominous sounds of his breathing...
Yes, Vader may be dead and "redeemed" but that doesn't negate what he did and the emotional toil he had on the galaxy, Luke, especially. I could see it coming back to haunt him in this trilogy. But that's the only way I'd really think it'd be appropriate for Vader to "return".