Oooohhh boy, you just touched on one of my ultimate tinfoil hat theories. Now I can't just leave that alone can I ?
The idea of at least Sora and Kairi being separated over and over again is something that was touched on very heavily at the end of KH3. Sora even goes as far as to say the ENTIRE reason for his journey is so that he can be together with Kairi again, but every time it gets close to happening, she slips away again.
In remind, Nomura doubles down on this concept having Sora repeat the line that saving her is the whole reason for his journey.
Now, having this concept in your head, let's examine Sora's quote at the beginning of KH3.
"They can take your world. They can take your heart. Cut you loose from all you know. But if it's your fate, then every step forward will always be a step closer to home."
This quote I think is meant to give us the theme that we are not in control of our own destinies, that everything has already been decided and events are fated to happen. Though it gives a positive tone that ultimately fate will lead us to where we belong, despite all the negative things that may happen along the way.
Now with that out of the way, let's get to the theory shall we ?
I believe this quote is a line that Sora will at some point in the future speak to the Master of Masters.
The Master of Masters I believe represents the futile struggle against fate. Here is a man who traveled back in time and went through great lengths in order to see the future and plan accordingly in order to alter destiny. Looking at Sora's quote, you could say the Master's ideals are in stark contrast to Sora's who believes that fate will ultimately lead to salvation. It's no wonder that these two end up being pitted against each other in the next saga. So how could it be that these two have come to live such different lives that they have come to believe such opposing viewpoints. This brings us back to Sora and Kairi.
Imagine for a moment that Sora did not save Kairi at the end of KH3. Imagine that he did not use the power of waking to save the guardians, imagine that Darkness did in fact prevail and Light did indeed expire. What would this world look like ?
First of all, Sora would be alone. Without his friends to give him strength, how could he continue to fight ? Would he simply give up and allow himself to be defeated by Xehanort ? I don't think so. Remember that Light expired, so what does that leave in this world ? Darkness. I believe Sora in this world would turn to the power of Darkness and seek revenge against Xehanort for everything he had lost. With the Guardians of Light defeated, Xehanort would be forced to enact his backup plan to use the Princesses of Heart to forge the X-Blade instead. Xehanort would quickly retreat with his 13 Darknesses assembled, and have each of them clash with the Princesses of Heart instead, ultimately extinguishing the last light in the world, forging the X-blade and completing his goal to summon Kingdom Hearts and purge the world back to complete darkness so that the world could be reborn anew.
There's only one problem with this plan, Sora is pissed!
Sora arrives just before Xehanort is about to complete the X-blade, with only one Princess of Heart left to form the final key ... Kairi. Sora watches in shock as Xehanort strikes her down, not crystallizing her this time, but actually killing her. Upon seeing this, Sora's rage becomes uncontrollable and fully succumbs to the darkness transforming him into his rage form. The battle between Sora and Xehanort plays out much the same as it did in KH3, with Sora mindlessly decimating all of Organization XIII, including Xehanort. He claims the X-blade and the power to control Kingdom Hearts for himself. However, with all the light in the world having been extinguished, Kingdom Hearts no longer had any power and the X-blade was useless.
For years, Sora wandered this dark world alone, wondering what he could have done differently. How could he have failed so miserably as for things to end up this way. He cursed his fate, and slowly ... the darkness began to drive him mad ... he had forgotten that deep down ... there's a light that never goes out.
One day, after countless years of wandering, a dim yet blinding light emerged from the darkness. Just as the legends had described, the first light was born. Kingdom Hearts was reborn.
Sora awoke before its glowing brilliance dumbfounded and speechless. Yet he could feel its radiance somehow compelling him to move forward. He looked down into his hand and felt a weight he had not felt before. The X-blade too had awakened. He raised it towards the now enormous light before him, and in his heart he knew what it was he had to do. He would go back, far back, as far as he could go, and undo everything that happened, every single event that led up to this cursed fate. And he would make sure that he couldn't fail. He would devise a plan to see into the future so that no matter what, he would always be one step ahead.
This time, he and Kairi, and all of his friends would finally be together, and there would be no way that fate would be able to stop him ... he turned the X-blade and blinding light swept over him as he muttered the words to himself "May my heart be my guiding key."
So there you go folks, my crazy head canon. Yes I am one of the people who thinks Sora is the Master of Masters, and I think I've laid out a pretty good possible scenario as to how that would be the case. If this turns out to be correct, I think it's an excellent story. What better conflict than for the protagonist to ultimately be forced to face off against a version of himself who simply was just dealt a bad hand in life. It's a story of how a person can possibly become completely different and have completely different views just as a result of pure happen stance. On the one hand we have a hero who embraced fate, and on the other a villain who despised it, yet both desiring one simple thing, the love and companionship of his friends.
I personally would love to see this story play out, and see how Nomura would end it. Will Sora be forced to kill himself, or find a way for both versions of himself to be happy in the end ? I might never get to find out, and this might be a story that only exists in my head, but I still love to think about it all the same.