Late reply, but I'd like to add in my two cents! So I'm sure some of your initial opinions and assumptions have changed since it's been a while, but I would like to clarify some things. Ephemer's lifeboat failed - he was the only one who was still inside their's, and he woke up in the ruins of Daybreak Town before turning it into Scala. In Kingdom Hearts 3, when Xehanort introduces Scala as the nexus from which all worlds spring, it's because the world is the same (it's Daybreak Town's world), but there's just a different civilization there. What exactly happened with Brain is up in the air, but based on Back Cover, which gives further insight into the Foretellers' perspective, it's reasonable to assume that Luxu sent Brain's heart into the future and snatched his empty body to become his first vessel in order to save Brain. Brain then wakes up in Scala. While we don't have any clue who Xehanort's family is/was, it has been confirmed apparently that Brain was actually Eraqus' ancestor, hence their many similarities. We have no idea where Skuld ended up, but it's a popular theory that she's Subject X, the girl Lea and Isa were talking about searching for in KH3. Ventus seems to have been picked up by Xehanort, who was able to tell that Ventus was a keyblade wielder, however Xehanort mistook the darkness in Ventus' heart as having come from Ventus himself because it was so intrinsically tied to his heart. We know this wasn't the case; the darkness inside Ven that became Vanitas was the darkness that possessed him in the ancient past, so on that front Xehanort was actually mistaken in that darkness wasn't Ven's true nature. Lauriam and Elrena woke up in different Disney worlds, and it seems that they never learned they were keyblade wielders before joining Organization 13, as they were surprised to hear that they had connections to the ancient keyblade legacy in KH3 from Xemnas. It seems like, only now after KH3, does Lauriam and Lauriam alone recall the events of UX out of him, Elrena and Ventus. It seems like Elrena and Lauriam did have some sort of inclination towards each other even without memories though; they not only were partners in crime working to stage a coup within the Organization, but Larxene rejoined the Organization just to go along for the ride with him (it's heavily implied, at least). The next arc of Kingdom Hearts seems to be focused around the Foretellers and the ancient keyblade war, and it makes me believe that the lost new union leaders will play a major role. While I believe that Brain and Ephemer are most likely dead by present day in KH, I wouldn't be surprised if we found out that they reincarnated, or if they possibly time travelled (again) to help out. One theory I'm a fan of is the idea that Sora is Ephemer reincarnated, however there's no real way to tell, as Xehanort as seen in Dark Road is a special case of retaining some memories of his past life.
Aside from your last theories, I had pretty much figured all of that out, but I still appreciate the effort! The theory that Ephemer reincarnated into Sora is an interesting idea, but as far as we know, the only way for that to happen is if someone is in a world when it falls asleep and they choose not to sleep themselves. While it's not impossible, I'm not really sure if Ephemer would end up in another situation like that. I guess we'll find out as the series continues!
Late reply, but I'd like to add in my two cents!
So I'm sure some of your initial opinions and assumptions have changed since it's been a while, but I would like to clarify some things. Ephemer's lifeboat failed - he was the only one who was still inside their's, and he woke up in the ruins of Daybreak Town before turning it into Scala. In Kingdom Hearts 3, when Xehanort introduces Scala as the nexus from which all worlds spring, it's because the world is the same (it's Daybreak Town's world), but there's just a different civilization there. What exactly happened with Brain is up in the air, but based on Back Cover, which gives further insight into the Foretellers' perspective, it's reasonable to assume that Luxu sent Brain's heart into the future and snatched his empty body to become his first vessel in order to save Brain. Brain then wakes up in Scala. While we don't have any clue who Xehanort's family is/was, it has been confirmed apparently that Brain was actually Eraqus' ancestor, hence their many similarities. We have no idea where Skuld ended up, but it's a popular theory that she's Subject X, the girl Lea and Isa were talking about searching for in KH3. Ventus seems to have been picked up by Xehanort, who was able to tell that Ventus was a keyblade wielder, however Xehanort mistook the darkness in Ventus' heart as having come from Ventus himself because it was so intrinsically tied to his heart. We know this wasn't the case; the darkness inside Ven that became Vanitas was the darkness that possessed him in the ancient past, so on that front Xehanort was actually mistaken in that darkness wasn't Ven's true nature. Lauriam and Elrena woke up in different Disney worlds, and it seems that they never learned they were keyblade wielders before joining Organization 13, as they were surprised to hear that they had connections to the ancient keyblade legacy in KH3 from Xemnas. It seems like, only now after KH3, does Lauriam and Lauriam alone recall the events of UX out of him, Elrena and Ventus. It seems like Elrena and Lauriam did have some sort of inclination towards each other even without memories though; they not only were partners in crime working to stage a coup within the Organization, but Larxene rejoined the Organization just to go along for the ride with him (it's heavily implied, at least). The next arc of Kingdom Hearts seems to be focused around the Foretellers and the ancient keyblade war, and it makes me believe that the lost new union leaders will play a major role. While I believe that Brain and Ephemer are most likely dead by present day in KH, I wouldn't be surprised if we found out that they reincarnated, or if they possibly time travelled (again) to help out. One theory I'm a fan of is the idea that Sora is Ephemer reincarnated, however there's no real way to tell, as Xehanort as seen in Dark Road is a special case of retaining some memories of his past life.
Aside from your last theories, I had pretty much figured all of that out, but I still appreciate the effort! The theory that Ephemer reincarnated into Sora is an interesting idea, but as far as we know, the only way for that to happen is if someone is in a world when it falls asleep and they choose not to sleep themselves. While it's not impossible, I'm not really sure if Ephemer would end up in another situation like that. I guess we'll find out as the series continues!
Hello! do you have a discord? I would like to discuss some interesting HOH Theories with you, after rediscovering this series again.