What do you think is the differences in souls?

[Lyth]

I don't think that souls in this particular in-game universe behave the same way that many of us would intuitively expect. Your sense of 'self' is largely a function of your memories, and most memories don't remain on a soul after it's returned to the lifestream without a good reason (i.e. powerful unresolved regrets, or Kairos' aetheric manipulations). Although I wonder what happens to all these memories. Perhaps there's a lifestream primal like FFVII's Minerva who sits around jotting all of them down somewhere.

Either way, if a soul does come back and there isn't one of those extenuating causes for retained memories as above, then it's a completely new entity. Who knows, perhaps a dragon soul might just spontaneously fragment in the Lifestream and spontaneously turn into 14 Elezen the next time around. I don't think that the native/alien distinction really matters either way, because it's just aether at the end of the day. I seem to recall something in one of the UT cutscenes that talks about lifesteams in other worlds, but I can't track it down at the moment. I might have a look later.

[Jandor]

I don't think that souls in this particular in-game universe behave the same way that many of us would intuitively expect. Your sense of 'self' is largely a function of your memories, and most memories don't remain on a soul after it's returned to the lifestream without a good reason (i.e. powerful unresolved regrets, or Kairos' aetheric manipulations). Although I wonder what happens to all these memories. Perhaps there's a lifestream primal like FFVII's Minerva who sits around jotting all of them down somewhere.

Seems that you can take memories in, you can't take them back out (beyond something creating a stain too stubborn for the lifestream bath to remove, like the memory equivalent of red wine on a white shirt.)

I did get the impression that you retain your memories and your sense of self, and possibly all your previous memories from other lives as well, while you're in there. Moving on/sinking deeper into the aetherial sea seemed to be at least somewhat of a choice, and you could choose to hold off on it (at least for a bit) if you wanted to.

Beq Lugg notes that the rest of the Scions have denser souls than citizens of the First, so they still got another piece added back after every Rejoining. My theory is they're the product of the souls born after the Ancients began sacrificing to Zodiark but before the Sundering. As the Echo is awakened by a soul remembering the trauma of the Final Days (well, the first Final Days), they can't awaken to it because they were never there.

It's possible that a soul is just a soul and they all start out at equal density, it's only Etheirys unique split nature that results in different densities on the different worlds.
Essentially, whatever process it is that leads to the creation of a soul is not sundered, but the process that places said souls into living beings is.

In theory a fully rejoined but not fully restored world would see all the sundered being born with proper full fat 14/14 souls.