Recently on the app page you mentioned that Starscream's immortal spark would have been converted to a 'human' soul... however, I don't see why you can't just have the reincarnated characters' souls/spirits/sparks/whatnot still being whatever they were in canon, but like... with Earth!human limitations applied, instead? In this case, the echoes are simply unlocking certain abilities that their soul or soul equivalent would have allowed them if they were in their original world/canon.

This is an important distinction to me because, in Final Fantasy XIII's universe, humans' spirits are made of a mysterious force known only by the description of 'chaos.' It's what defines the unpredictability of humans... and is what gives Lightning her power since she's particularly close to the goddess who accidentally created humans but then deliberately placed pieces of the chaos inside them. Lightning is very in-tune with her own spirit, which is what allows her to do magic and eventually be indestructibly immortal in XIII-2.

Anyway, having to use an echo to change one's soul to whatever it was before seems unnecessary to me and, if solidified as a thing, I'd probably have to change Lightning's initial echo to either include that or be something else entirely, given her supernatural abilities are inextricably tied in with what her soul is made out of. It just seems easier to say limitations are gradually being lifted rather than trying to figure out who does or doesn't have soul-powered abilities and then whether or not said soul is different than everyone else's...