WARNING: RAMPANT SPECULATION AHEAD
In regards to Eden, specifically E8 and the stuff contained therein, something always bothered me and I think I might have put my finger on it:
What if Ryne and Ysayle were shards of the same original soul? What if - through the magical aether control fuckery of Eden, the WoL actually drew out Ysayle's soul from the Lifestream when conjuring forth the image of Shiva? Shiva was unique among the primals in that she both required a host and did not subsume the will of the host towards the goals most primals tended to pursue. Shiva was also unique in that she was a primal-like entity, summoned via primal-like methods, but all contained within an entity with the Echo, something that was (at the time) believed to be the antithesis of primals and their powers.
If Ysayle and Ryne actually do share a soul, it would go at least partway to explaining why Ryne lost control during the fight. Ryne may be young, but she's no slouch; she was able to control and seemingly build almost a relationship with Eden as an entity, using it to do some wondrous things without any kind of negative feedback or consequence. It stands to reason that having seen the WoL face off with 5/6 primal forces of nature, Ryne surmised she had it in her that she could handle that amount of power. Of course we can't discount the confluence of two similar elements like ice and light either, but I don't think that's the end of the story per se.
Now, I know what you're thinking; what about Minfilia? She existed on the Source at the same time that Ysayle existed, therefore they can't be the same soul. And I would agree with that, but I would also note that Ryne is not Minfilia. Minfilia stated in our final meeting with her that she was tired of taking the lives of all these girls and subsuming their souls with her own. She wanted Ryne to be able to choose her own destiny as her own self, and instead of taking over Ryne's body with her soul, she instead gifts her powers to Ryne and leaves her soul intact with Minfilia's soul finally moving into the Lifestream. This allows for the possibility that Ryne and Ysayle could be two shards of the same soul, much like the WoL and Ardbert.
My last bit of evidence comes in the form of these two passages from "Return to Oblivion."
Falling too far
Feel no fear to embrace me
A voice from the past
Screaming, "There is no end now"
A slave to my fight
Am I doomed to repeat this
Again and again and again and again
I'm falling to fast
Now the ground overtakes me
A voice from the past
Echoes out like a drum roll
There's no more goodbyes
And my heart is still aching
I'll open my eyes
One more time
One more time
I believe the first passage refers to Ryne, and is basically being "sung" by Ryne. It jives with her past, as the inheritor of Minfilia's powers and memories, she essentially has the experiences of all the previous incarnations of Minfilia, each one of which fought a losing war against the Sin Eaters until they died, for the past hundred years. Lines like "a slave to my fight" and "am I doomed to repeat this?" lead me to believe that Ryne is having a moment of clarity during the fight where she's aware of what she's doing but is incapable of stopping it. She's afraid she'll trigger another Flood and continue this insane cycle she's been incarnating into.
But the second passage diverges a bit. The imagery evoked in it doesn't fit so easily with Ryne and her experiences as Minfilia's successor. It does however fit pretty well with the final moments we see with Ysayle, as she's basically shot out of the skies around Azys La and plummets to the earth mortally wounded.
"A voice from the past echoes out like a drum roll" this is the WoL calling the power of ice to take form as Shiva, and Eden manifesting it's power to draw forth the latent aether of the First. In the process of this, Ysayle's soul wakens from the voice of her past calling out to her as the physical embodiment of ice.
"There's no more goodbyes and my heart is still aching" is Ysayle - now essentially reincarnated as Shiva - but sharing a body with this girl she's never seen before. A girl who is on the verge of panic because she's about to destroy the world she's tried so hard to save.
"I'll open my eyes one more time one more time" is Ysayle as Shiva resolving herself to whatever fate has plucked her from the afterlife and facing it with her eyes open and head held high. Upon seeing her old friend and comrade in arms, she loses herself completely at the insanity of it all - truly becoming Shiva the primal - and in the process causes Ryne to lose herself as well.