While I don't have a concrete plan yet, I would still like to propose this as a concept pitch to anyone who might be interested in taking part.
The game will be set in the "Birth by Sleep Era". In other words, a time when Keyblade Masters were still teaching apprentices. Though unlike Birth By Sleep, this will be set 60 years before KH1 instead of just 10. As such, many familiar characters will not be appearing because this will be happening before their time. The basic gist is that in The Land of Departure, Master Yen Sid (Who would still have been a Keyblade Master at the time) is preparing his most recent pupils for the Mark of Mastery Exam when he receives urgent news. A man by the name of Malcolm has reportedly returned and this is worrysome enough that Yen Sid postpones the exam and, for the first time since the Keyblade War, unlocks the Lanes Between so that the threat can be dealt with.
As for who Malcolm is and why he's such a big deal, here's the rough draft version of the story. 25 years prior to the events of the RP, the Keyblade wielders were thriving, using The Land of Departure as their home world. Back then, they called themselves the Keyblade Society, and they governed The Land of Departure with a Council of Keyblade Masters. One of the Councilmen, Malcolm, abruptly leaves The Land of Departure by means unknown after he attacks and kills the other members of the council. Once departed, Malcolm begins experimenting with the forbidden power of Darkness, infusing it with various test subjects and eventually creating a force of creatures that only he could control. Using his new creations, Malcolm launched an attack on the Keyblade Society. With no council, the Society was forced to elect one of their Keyblade Masters as King so that he could use the authority thereof to lead the Society in what came to be known as the War of Souls. Malcolm's new creatures appeared to possess no hearts or minds, nothing that could identify them as anything other than pure abominations that the Society soon dubbed The Soulless. After a series of fierce battles, the King finally managed to put down Malcolm, though the exact circumstances of that battle have remained a mystery. All that is known is that both Malcolm and the King were destroyed in that battle.
So yeah, Keyblade Master gone dark threatened to destroy the Keyblade Society 25 years ago, was apparently stopped, but is now somehow returned with his creatures the Soulless. In the end it will be up to a group of Apprentice Keyblade Wielders to discover the truth about the war and put a stop to the mad man Malcolm once and for all.
Before I end this post, I feel the need to warn everyone that the nature of this setting means that none of the previous worlds can be visited, since none of those characters or stories have come around yet. That means we'll need to work together to come up with some new ideas for worlds to visit. Hope you're up to the challenge, and I hope to read your responses soon!