Or, since we have already basically killed all of the canon, perhaps some other hacker, maybe Fairy King Oberon/The insane dude from the Alfhelm arc installs a virus into SAO in an attempt to take over, in an act of rivalry and stuff, Kayaba reveals himself to the United Swords and enlists their help in a "Lesser of two evils" kind of things. After looking at their selfless sacrifices to save eachother, Kayaba finally feels the one thing he never had, anger, sympathy, regret. Anger that this fool would enter his game and attempt to claim it as his own, before attempting to murder all of the people he had come to know as friends, sympathy to the rest of the players, finally understanding what it was like to watch your entire life burn up in front of you, and regret that he made the death game in the first place. Deciding there is only one way to truly gain redemption for his sins, he sacrifices himself to save Anicrad and its inhabitants. From there, we could branch out, saying that now the ghost of Kayaba, downloaded onto the internet, visits some of they key players in Anicrad, enlisting them once again to take down that other dude from Alfhelm, before traveling the internet multiverse, facing off against foes like Death Gun and Hackers.