I thought this was worth discussing, and I'd love clarity on things incase I'm misinformed, etc.
I think the whole idea is brilliant but what else is new from Nomura. Confusing as always, but with purpose.
The events in X being in the RoS seem to make the most sense, and functionality-wise, it's a great way to remove "us" from the story. Since the main character is essentially a player avatar, we couldn't possibly have a place in the storyline for III right? If Ephemera is from Unchained/the real world, he can definitely reappear in the story (most likely will). I do wonder what "our" fate will be, but I'm sure we'll wake up at the end of KH3 and go one many online multiplayer battles and missions blah blah blah
Another thing to think about, is taking the names literally, and sort of applying the title I have this topic as a possible plot point. Throughout the series we've been given this idea of keys clashing. Sora vs (possessed) Riku. Riku vs Roxas. Ventus vs Vanitas. Even upon Unchained being revealed, we have to fight Master Ava. The X-Blade has to be forged with keys clashing. Blah blah blah you get the idea. What if the world of X and the world of Unchained clash, thus creating the legendary war? Ava did say Ephemera was closing in on the truth of their plan...
I'd also like to point out that it is always light vs darkness, even in this example. Even Terra vs Aqua for the Mark of Mastery was a foreshadow of the future fight. Maybe Ava really is the traitor? Or she could just represent all the foretellers? Maybe I'm reaching? Lol.
But yeah, That's pretty much my theory.