Here's my AU (using post 1's format cause I like the format)
Fall of Heroes
Locations: The Great Spirit Robot, Aqua Magna, any island on Aqua Magna
History: The Toa Metru fail in their task to rescue the matoran of Metru Nui and are forced to leave, escaping the Great Spirit Robot and making their way to the other islands. The Toa Mata fall to Mata Nui and find Mata Nui empty, by the Bohrok. The Makuta recruits the Toa, mainly to eliminate any chance of his failure. During his conversion of Toa Mata, the Makuta species were attacked by a plague that weakened the power they could access. The Toa Metru recruit many others on the other islands and a war breaks out, during this new Toa rise to face the Makuta forces. Karda Nui became a permanent warzone as both sides constantly fought over possession of it.