If it were ever possbile, how could Sephiroth (Final Fantasy) come back to life? Possibilities and how?

Okay, we all know that Sephiroth was killed twice. When dealing with the second attempt, in Advent Children, Cloud killed him for good. But for wonder sakes, can you think of any possibilities on how he might be able to come back? If you do, can you go into detail on how? Any ideas are welcome.