I've had this headcanon/theory rattling around in the chasm between my ears for a long time now. Basically, I believe that Aqua was unknowingly a princess of heart before/during the events of BBS.

It clicked for me on a random replay of her campaign after one of the fights with Vanitas where he says "Never hurts to have a backup," referring (I'm assuming) to her being a replacement for Ven in the attempt to make the X-Blade. Based on Xehanort's own logic, that wouldn't be possible unless she had a heart of pure light. It might also be why she could tell that Kairi had a heart of pure light.

At the end of the game, as we all know, Aqua gets trapped in the realm of darkness after trying to save Terra in Radiant Garden. My belief is that after a few years being trapped there, darkness managed to take its toll on her and cause her heart to no longer be pure light. That pure light then transferred to a new heart, particularly Alice, as i looked it up and found that she is canonically around 7 years old during the events of her story, meaning she would have been born while Aqua was trapped.

This idea came full circle for me when KH3 confirmed that Princess of Heart status does transfer between people for one reason or another. It can even be choosy as to who gets to keep it and who doesn't, since Kairi is still one of the princesses of heart during KH3.

Thoughts?

TL;DR: I think Aqua was a princess of heart but didn't know it, and was replaced by Alice while trapped in the realm of darkness.

What if Aqua