I think that the final battle against Master Xehanort should to happen inside of Kingdom Hearts's moon that also can to be the final world of KH3. In the moment that Sora & cia reach the moon's core, the real Kingdom Hearts, Master Xehanort will to have started at assimilate KH in himself, and to prevent Sora of stop him, MX will to summon "No-Heart" to fight Sora beginning the 1st part of the battle and after defeat the keyblade armor, Sora will to try attack MX but of nowhere one strong light envelop Xehanort and the moon will to begin at shake forcing Sora and the others at flee of the moon and after the escape, the entire moon will to "disappear" in one very strong flash of light, causing the characters think that Xehanort died, but of nowhere, one child's voice call their attention and them see one new Master Xehanort, who appear have one body of 14 years using one grey coat, as one symbolism of light and darkness's fusion, and some detail in his face as one heart's mark on the forehead and the new and younger Xehanort confirm that he and KH are one and the same, and after of more talk, Xehanort throw Sora of way that he will to stop in his home world and Xehanort will to appear in his ahead and after more talk related at the past of Xehanort, he summon the x-blade and the final battle finally to begin, but Sora no will to cause damage in the "Ultimate Xehanort", and after one little time, Xehanort will to throw Sora for another world, that is the Castle Oblivion, where Xehanort will to talk more about his past with his former friend Eraqus and after more some time of fight, Xehanort will again throw Sora at another world related at the past and after more talk and fight, Xehanort will send Sora at the Keyblade Graveyard where Xehanort will to finally to rid of Sora and the others but something will to happen and finally Xehanort will to lost his special defense and will to receive damage, finally beginning the true final battle of KH3.