Hi I have just completed my second playthrough of KH3, and have some new insights and speculations I would like to share. I don't have friends to discuss KH3 with irl so I'm so very excited that I have found this forum. So here goes.
Regarding Sora's 3 compartments, I thought that they were Roxas, Ventus and Xion after my first playthrough. But after my 2nd, I think the third one might not be Xion. Throughout KH3, hearts have escaped Sora's body twice when "sparks" were introduced, in Ventus's case it was when Vanitas was about to strike down Aqua; and in Roxas's case it was during the Org13 fight with Saix and Xion. This means that there is still one more non-Sora heart within Sora, and according to the Secret Report it is one that has "held its heart much longer than Ventus's did". This is unlikely Xion, as Xehanort and the Org13 was able to create Xion and put her into a replica without extracting her from Ansem the Wise's data OR from Sora's heart.
I kinda think that this third heart is either inherited by Sora by birth, or seeked refuge within Sora like Ventus's heart did, long before Sora was a kid. And I guess that this heart will link Sora to one of the KHUx character. This would be a cool way to keep Sora relevant in games to come. My guess would be Ephemer since Sora was able to summon the help of ancient keyblade wielders during the final KH3 fight (the flying keyblades), and this seems to be connected to Ephemer since he was shown before Sora was able to summon the flying keyblades. Also this explains Vanitas's battle move that summons flying keyblades that he can ride on during the Org13 fight, coincidence?
However, I have another theory that counters the one above. Regarding the time travel part, it is known that 5 characters and a key has travelled from fairytale time to the BBS era and have been keeping up appearances in the recent stories, and this was the plan of Brain who was the imposter who killed Strelitzia. My guess is that Luxu has bequeathed No Name to Ephemer (whose descendant would be named similarly - Eraqus, starts with E - and in place to inherit No Name). This should mean that Ephemer wasn't one of the time traveller as he was to lived his life in the post-keyblade war era in order to pass on his No Name generation by generation, Luxu was to make sure this happen. This would oppose my guess that Ephemer's heart finally ended up within Sora's.
As for the time traveller, my guess would be Lauriam Ventus and Elrena (this would tie the long due question of why these characters appear in both fairytale time and current time), Brain (he is the initiator) and Skuld (there has to be another girl who Lea and Isa cannot recognize, specimen X in secret report). Other candidates could be Demyx and Luxord, who was revealed to possess "ancient keyblade legacy". From Luxord's last words, I'm pretty sure we will see more of him in the next game. Maybe there will be multiple batches of time travellers besides the first 5 mentioned by specimen X?
I'm very excited to see how the story progresses. I'm thinking that when MoM "vanished without a trace" in Back Cover, he is actually summoned by Xigbar from the future. And the reason he wants to be present in that future where the scapegoat fails, is because he wants to see Sora, the person that brings light back even when darkness has taken over. What happens beyond this point would not be recorded in the Book of Prophecy anymore, so the MoM is no longer a "foreteller" so to say, just a keyblade master that has divine power.
What do you think? I know there's a lot of extrapolation, but this is what I think at this point.