We have fought the Void many of times. We have stormed the Seat of the Triumvirate with Alleria, and delved into the depths of the Mana-Tombs and Auchenai Crypts.
The Void has always been the "evil" of WoW, and for good reason but as we have seen with the introduction of Void Elves and power of Shadow Priests it is not only used for evil. The same could speculated for the Light. In WoW there is the Scarlet Crusade that uses the Light in a Zealot style way but they are barely more than an annoyance.
Within WoW the Light and the Void must exist as one cannot be used without the other. It would be nice to finally see the effects of those that follow the Light a bit too much. The individuals that believe that there is only one purpose for the Light and plan to assault all those that "go against" the Light, whether it is Mages and their control over Fire, Frost, and Arcane and a War against the Shadow Priests that use the Void for good.
It would be a very interesting change to see how important the balance of Void and Light are and that they both can be as evil as the other and as useful as the other. (And yes I do know the Void inherently whispers Madness and caused the Old Gods making them the big bad but it would be nice to see how the Light can be twisted as well).