As I mentioned before, Xion wasn't apart of Days until the game was already in production. Taking that into consideration, and looking to KHII for context, I feel that Naminé was the girl Nomura had imagined Roxas left the Organization for, and the third piece of the Nobody trio. This would also better contextualize Naminé's actions and feelings during the Roxas segment of the game, as well as the scene with them on the top of The World That Never Was, and during the ending.
As a Naminé fan, I'm pretty upset about that.
Couple of months late, perhaps more, but as I filter through threads to entertain me I feel you have a very palpable point. Xion was created and added [unnecessarily so] by someone else in charge of the novels, and wasn't thought of at all by the director himself. Which is why they had to literally clean traces of her for
KHII to be near sensible (
KHII has its own issues, nonetheless). The reason why I'm replying is that while Nomura was more pegged as a scenario writer, it feels like he still managed to include shadows of what his original idea
may (keyword) have been. This is in theory, but...I have
Days, and shifting through it, as well as the HD remix, has your statement about Naminé being the ghost girl Nomura had in mind for Roxas leaving more and more plausible.
Despite the incredibly forced Xion insertion, Naminé's presence was
still included when it came to his character. She sort of played the mysterious "woman in white" persona in his point of view. He's seen her in his visions (something that occurs in
KHII as well), and in the scene when he deserts his friendship with Axel, it sets up the potential ties for Roxas towards Naminé later on with this simple, penultimate line:
"Somebody knows where I came from. If I can't get answers here, I'll get them somewhere else. That'll be the person...I trust."
All in all, Naminé, even with what
Days turned out to be, was situated to be the one to reveal what Roxas' true self is.
I first thought that it was a shallow shipping dialogue to sort out
KHII and smooth over why Roxas and Naminé seemed close despite meeting only briefly, and that's more than likely the case. But it gives credence to your assumption. They're both my most favorite characters (next to Larxene, don't ask), so that would make up for Roxas' lack of character action and purpose in the game and I would have gotten to see more Naminé as added bonus.
I would have preferred this so much more. The potential and how it all would have connected is a loss. Yet I can't be angry at
Days for what it
could have been anymore. It's not going to change anything.