I have a theory about this.
Paradox.
Things that aren't meant to be there (aka the world of FEF)
Noteworthy explanations.
Luna, Odin, Not!Inigo. (They all looks so like the 3 characters from Awakening, even Luna has the same voice actor)
Lucina, Marth, Robin, Ike amiibo keeping their ideal self and logic, very unlike Einherjar who doesn't know they're false copies. [As proven by Lucina's dialogue that a very devoted translator translated for us that said she came from her fate to see us(Kamui)] (They're not in this world, rather they all come from their own worlds yet they came here, Paradoxical meaning.)
Aqua's tendency to suicide via her expression and feelings (Proven by her trying to drown herself, and uncaring for her life when trying to settle down Kamui's berserk state during dragonform.) To be explained later down.
Purple Energy on Aqua's arm for a wound and around the bad hooded guy in trailer.
Why do I believe they're all paradox?
Well, because I played 13-2/13-3 of the Final Fantasy series so I know a good deal about Paradoxes.
Let's expand into that particular realm.
We all know Time-traveling re-use would be a stupid story element, and I'm certain IS know that. So it won't be a thing, which is what I believes.
Paradox, to be more defined, is a term used to describe something that should NOT be there, for example let's take a more loose idea. A gun in ice age. That would be a paradox, they're simply not meant to be there, men hadn't discovered it. Perhaps an alien planted it there? Or it was warped into a wormhole ending up in Ice Age? Or it's simply an illusion that never ends.
That's probably a poor example but nonetheless the concept is there.
Let's explore how paradoxes can be in FEF without the obvious look-a-likes.
Aqua.
I believe she's, to a loose form, a paradox in the world of FEF.
Why?
1. She follows Kamui for no reason, she doesn't even know him, yet she follows him even if it means betraying Hoshido where she was raised in.
This expands into the next point.
2. She's never shown to be talking to ANYBODY BUT KAMUI. Does this means other siblings doesn't even bother recognizing her, or perhaps she's just as unknown to them as they are to her? Which THEN expands into the next point.
3. Aqua is very likely to be a part of Kamui, that wasn't meant to be split from him. If you remember, Aqua is a princess of Nohr kingdom kidnapped at a young age. At the same point, Kamui was kidnapped at a young age as well. Which raise a question.
Is Kamui or Aqua really related to either kingdom siblings?
If third point is true, then no matter what, their separation occurred at birth.
In the other word, the Light and the Dark Dragon avatars are one and the same entity but split by unknown means, leading to my speculation that it's caused by the strange purple energy that seems to emit from that guy, and the energized wound on Aqua. (which no doubt is either healed or sealed away.)
However this does not confirm whether or not that Light and Dark dragons themselves are a same entity as well, for all we know, they could be different side of a coin opposing each other unseeing each other and may be very well the final boss of neutral route where Kamui decides to explore his origin with Aqua while stopping the war ending up fusing with her and obliterate the positive and negative dragon from existence leading to unity that we all adores and loves in both kingdom and Camilla can finally husbando Kamui.
Edit:
I believe Aqua is simply Kamui from alternative world of FEF, as you know two same entity cannot exist on same plane.
E2: This is why children from another "future" are able to enter this world, Parallel world will track each other, but one is always ahead.
Oh boy, this makes my brain explode now as I think more.