It's my theory that there's two kinds of souls, now. "Old souls", which are fragments of shattered Ancients' souls, and "New souls", that are borne from the Lifestream. Old souls are incompatible with the Lifestream, and rather than dispersing, just hang around until they're born into new bodies.

For new souls, the USUAL thing is for them to disperse upon death. Occasionally, however, they manage to remain intact after death, and can even form physical bodies for themselves. The Scions did this, essentially forming bodies exactly like their originals. The Pixies are the result of this, as well, and likely ghosts are a sort of half-assed version of the process.

Because Ascians souls aren't compatible with the Lifestream, I also think that the Lifestream was created by Zodiark, and that it, and all of the life that emerged from it, was the life the Council of Thirteen hoped to sacrifice to bring back their sacrificed friends.