Well nomura has confirmed that 3D will have to do with Riku and Sora leaving to go and take their Mark of Mastery exam and leaving Kairi all alone. So my theory is that Kairi will start becoming depressed and sad because Sora's gone. Then the newly reborn Xehanort (who is keeping his presence hidden) will convince Kairi if she becomes stronger she will able to help Sora and they will be together. So why would Xehanort go through all the trouble of teaching Kairi. Because when he was reborn he lost the ability to use the keyblade and if he can get Kairis heart he'll be able to use the keyblade again (kind of like how sora could use vens keyblade because his heart was inside him). I know this theory kind of dosent make sence but I would like to see something like this happen and it would make Kairi have more impact on the story.