Here's an idea I posted in another thread. Dont worry, no spoilers.
A lot of people have been talking about Keyblade wars and such so this got me think. Not exactly a war, but a battle. I say it all started centuries before the games, and was more about greed than hearts and such. I believe that the worlds were guarded by an order of warriors know simply as the Keyblade Masters. They remained fine until one of them became too intelligent and began to see the actual power he had, so he used it for personal gain. Seeing as the Keyblade feeds off of a strong heart, the Keyblade itself became demented and such. So it began to desire the Wielders dark desires, causing him to become much more destructive and evil, bringing out his dark side. Thus, a power struggle was formed in the group; a keyblade wielder wanted to rule all and threatened the boundaries of the world, which the rest of the Order could not allow. However, the immense power that the dark Keyblade Master had gathered from the misery and death of the living had caused him to out match the other Masters. The last three members eventually had no other choice but to seal him and his malice away in the world of the Kingdom Hearts, the once beautiful land in which the all originated from. Over time his dark heart slowly deteriorated and broke apart, thus the first true heartless creatures were formed from him.
To prevent this from happening again, The Keyblade Masters hid the keyblades from the world. However when the door was threaten to be opened again, they had to seek out a new keyblade master (Sora), to aid them in their cause without revealing their existence and cause even more questions about the door to Kingdom Hearts to be brought about. However, the threat that posed upon the door grew to strong for one keyblade master to handle, so more were chosen (Riku and the King) to stop the threat. Eventually order in the worlds was restored and all goes back to how it was; however, years later someone finds another keyblade... a dark one (not the soul eater or Anti-Keyblade) that had once belonged to the Dark Master. The person becomes consumed by all of his hatred, misery, pain, etc and decides to make everything around him feel his pain. Soon, the current keyblade warriors aren't enough to stop the strength that he rapidly gathers (absorbing his own heartless forces)... so they original masters must return from hiding and stop this once and for all. However, I doubt Sora and the others will let them kill another, no matter how destructive they are.
Some extra bit of info:
The reason I personally dont believe it to be an all out war is mainly because the term of War is very loose and can be applied to anything, so it is but mere speculation that a war (armies) of keyblade wielders will fight eachother. Remeber Nomura has an IDEA not an exact plot, so it is quite possible that whne he meant War, he meant a fight for the worlds between two wielders.
War Definitions:
(1) The waging of armed conflict against an enemy(s).
(2) A state of open, armed, often prolonged conflict carried on between nations, states, or parties.
(3) In an active state of conflict or contention.
Also, he says the third game revolves around this principle... "The legend says its wielder saved the world, while another says that he wrought chaos and ruin upon it."
