My theory is that in the next movie Sidious will be responsible for bringing Vader back to life by combining him with robotics and perhaps another person...Dooku to be specific. I don't have reasons to back this up, except I felt Dooku's voice and physique was VERY Vader-like when I first heard it, and I just generally like the idea of combining people with mechanics to make a more powerful hybrid Sith Lord.
<amalgam X>
Cheryl
lafah
Dadoftwo