Did you ever find it interesting that the prophecy mentioned in Star Wars Episode I: The Phantom Menace never really felt like it was, well, properly fulfilled? Traditional interpretations (found on Wookiepedia and various other fan-sites) would tell you that leaving Luke to teach the way of the Jedi was it; that was the fulfillment. I want to attempt to prove them wrong. I believe that Vader used Luke to bring about true balance.
Let's begin with the logical starting point: The Prophecy of the Chosen One. It reads:
"In the time of greatest despair,
a child shall be born
who will destroy the Sith
and bring balance to the Force"
From a traditional stand-point, we see that by Star Wars Episode III: Revenge of the Sith it would appear that Anakin Skywalker was meant to be that child. While this much is accurate, it would also appear as though he had miserably failed by succumbing to the Dark Side. This is where I think people start to miss the major point of the prophecy: balance. The Jedi had been handed down this prophecy from years ago and they took it to mean "balance is solely the Light Side of the force spreading peace throughout the Galaxy." THAT IS NOT BALANCE!!! That is heavily weighted on one side of the spectrum. Think of balance as the scale Lady Justice is always depicted holding, two sides each having the same amount of weight; equilibrium.
If the prophecy meant for the Jedi to interpret it with the "solely light side" mentality, IT WOULD NOT HAVE SAID BALANCE. It would have said something like "...destroy the Sith and bring peace to the galaxy." From a balanced perspective, Anakin truly fulfilled the prophecy.
By joining Darth Sidious, Anakin allowed for the execution of Order 66 and the mass murder of many Jedi, leaving behind Yoda, Obi-Wan, and several other unnamed stragglers. In the years between Revenge of the Sith and A New Hope, we know that Anakin (now Vader), hunted down and killed nearly all remaining Jedi. Now let's stop for a moment and take a quick count; Yoda and Obi Wan are the only two Jedi remaining, while Vader and Sidious are the only two Sith. That's pretty balanced, eh?
Well it WAS, until Obi-Wan had to go and take an apprentice. So it goes from 2 Jedi and 2 Sith, to 3 Jedi and 2 Sith, no longer a balance. In order to keep the balance, Vader kills Obi-Wan, bringing the count back down to a balanced number. Fast-forwarding to the climax of Return of the Jedi, Obi-Wan and Yoda are both dead and Luke considers himself to be a Jedi. This puts our count at 1 Jedi and 2 Sith.
Now this is the important part, you followin' me camera guy? Vader elicits a hateful and angry response from Luke, his son. This shows that, despite Luke's best efforts, Vader is able to turn him (even momentarily) to the Dark Side. In essence, by the end of Jedi, Vader and the Emperor have both died, eradicating the Sith, leaving just one: Luke the one and only Jedi... a Jedi who knows how to use and control his hate and anger (something Jedi are never supposed to feel).
What I'm saying is, in it's simplest form, Vader instilled knowledge of the Dark Side in Luke; this makes Luke a little of column A and a little of column B. Since Leia is unaware of her force sensitivity, Luke is the only one by the end of the Saga who has any force abilities. Since he's got a bit of both sides of the force within him (albeit one side is buried deep within him) he can be considered to be a balanced being; he is not solely good, nor is he solely evil. That sounds an awful lot like an equilibrium.
Through Luke, Anakin brought balance to the Force and destroyed the Sith. In a much more literal sense than the way Jedi interpreted the Prophecy, Anakin was in fact the Chosen One.
I don't know how Episode VII will affect my theory, but I am most certainly excited to find out! Thanks for reading!