There's an obvious difference between paying homage to the central themes and conceits of a work and rehashing the same premises and conflicts with some ornamental variations in context or character.
I don't think there's much of a difference at all. If you believe, it will happen. Everyone is connected and somehow, someway, you'll be able to do the impossible. Even the theme remains the same in Aqua's case. She's the light in the darkness and will somehow make it through even the toughest trials, whether it's a powerful boss heartless like Red Eyes or whatever his dumb official name was, or fighting within herself to overcome what she perceives as flaws about herself.
The core disagreement here still seems to be you think she's already been through that development and accepted that while I don't think she has. We can criticize the story overall all day for keeping information from characters for seemingly no reason (we have no earthly idea if they've decided to tell Sora everything like the basics of who Terra, Aqua, and Ven are so that he isn't surprised when he sees them. But since he doesn't even know Vanitas... well it's pretty clear they didn't), but the truth of the matter is Aqua doesn't understand that darkness isn't evil. That's not her fault. Her master told her that. Xehanort who prides himself on using darkness took out her best friends with it.
As a player, we know the darkness can be good because we've seen Riku's journey. She hasn't. She knows nothing about it. Maybe Ansem the Wise can tell her, I don't know. She hasn't met anyone other than Terra (and Ven I guess) who was inflicted with darkness and wasn't a bad person. Both of them are down and out. No one and nothing has presented her with a chance to truly think, "Oh wait, maybe darkness doesn't always lead to tragedy." yet.
My point is, with someone like Aqua, someone who believes darkness has essentially destroyed her life... right now she's in a shark den (the Dark Realm) with hungry sharks (the darkness). I know that sharks aren't the evil creatures they portray in Jaws that will attack anything with a pulse just because. But what about Aqua who believes every time a shark gets close to her it's trying to attack her, so she antagonizes it?
If you create the struggle (believe darkness is bad no matter what, resisting it to a point where the pressure becomes overwhelming), it will struggle and do harm. Of course we've all seen other characters believe darkness is bad and eventually come to the conclusion that it's not, but they haven't been swimming in a shark tank.
Sure, you'd like her development to be at the 200 mark, but Nomura hasn't advanced it past 150.
I completely agree with you that what we saw of the Dark Realm in 0.2 was mostly great, across the board. It generally felt like a proper expansion of the lore and mythos already surrounding it within the series, the only caveat to that being that while it kept in line with the understanding of darkness which has been reached through characters like Riku in an objective sense, it was all funneled through the perspective of a character who wasn't positioned to make proper sense of what was happening to and around her. That was dramatically gratifying but might have inadvertently perpetuated the mistaken construct of "darkness" as "evil" (because the series regularly associates it with bad things), and showcasing Aqua's corruption by darkness only contributes to that failure to clearly delineate.
I kind of already stated this above, but Aqua and Riku have lived out different experiences. No one has been in Aqua's situation. Unlike Aqua, Riku has the advantage of being in the light realm. While darkness was infecting him, he was still surrounded by light. It was a matter of taking control of his situation, realizing that little piece of darkness he had isn't inherently evil, and balancing things out. It's the exactly opposite for Aqua. Aqua IS the light and she's surrounded by what she believes is evil. The only person who can help her see the darkness differently is Ansem the Wise who has been through it all, but he doesn't even have all his memory so who knows what knowledge he could've imparted on her other than Sora's name.
My problem with this is that they're not doing anything inventive with her characterization, and yet this is being framed as a character moment for her. She's obviously going to be an antagonist of some sort who needs to be saved from the darkness, who's holding a grudge against someone for something-- that's a dime a dozen in this series, hell in this game alone we're likely to have several protagonists in the exact same situation. We can split hairs about what drove her to the darkness, sure, but if the end result is another assembly line possessed/corrupted boss fight with impermanent bad-guy affliction, it's a wasted opportunity to present the darkness as something different and Aqua as someone different in relation to it. KH3 would be the perfect time to take this road less traveled.
I still don't believe Aqua's 'grudge' will be a typical grudge. I don't think she's particularly out to get them and at the end of the day, she might not actually care about them. Even corrupted as she is, Aqua's goal might still be the same. Maybe while she's like this, somewhere the light within her still drives her to fox what's been broken (Ven and Terra). The only person I can really see Aqua 'going after' (not sure if antagonistically or just stalky and possibly confused since they look alike) is Sora who seems to not only has Master Keeper, but also Ven's heart.
Terra dabbled in darkness because of a lust for power and strength and later a bit of jealousy. Riku dabbled in darkness first because he was desperate to leave their world and then later out of jealousy. Ansem dabbled in darkness because he was betrayed by his students and wanted revenge and 'justice'. Aqua has never purposely dabbled and darkness, she's not jealous, she's not really out for revenge, she wants more than anything to be with her friends and somehow feels like it's her fault they aren't together. I'm basically talking in circles but... Aqua's situation just seems different to me. Completely personal and internalized. Even when she might trash talk someone else in the current state she's in, she probably still somehow it implies it's her fault they're against her in the first place.
The reason why I'm on the fence about her being norted is because it makes this possibility less likely AND it makes no sense why she's still just chilling in the dark realm. Surely she'd be out doing whatever the old man is scheming? I see a lot of story value in Aqua just having fallen to darkness and not being associated with Xehanort. So far, everyone who has 'fallen to darkness' is associated with Xehanort, but I'm trying to present a road where Xehanort is not the cause or reason (cue, being a Darkling/Black Costume)
I too am really interested in knowing what happens to a person with a strong heart of light who has remained in the RoD for an extended period of time. The thing is, we've already seen somebody who used the darkness to escape and carry out a plot for revenge/his own agenda. His name was Ansem the Wise, we got that plot line like a decade ago. (Yes, I know he wasn't there as long as she was, but I'm going to call that an arbitrary distinction here because again, what we're getting from her based on what we've seen is not notably different. He even masked his appearance using the darkness and got some new clothes.)
I'm not interested in a story where Aqua willingly accepts darkness or Xehanort just because she wants to get out of the dark realm. That just isn't her to me, she hates Xehanort. I find it more likely she was defeated by no one but herself. If Xehanort is involved, he only swooped in when she was already down and out like the scumbag he is. He already tried to trick her once so far.
I just cannot see Aqua shaking hands with darkness and accepting it. I can't envision her trying to play double agent or anything. Maybe it'll happen, but I can't see it. The only person we haven't seen purposely using the darkness to gain an advantage is Sora which is why I think Aqua situation would be more similar to his. Not the same, similar. (being force fed a darkness sandwich)
I'd actually argue that making her a "Darkling" or whatever diminishes the potential for this to be a character moment-- seems to me it's just another hackneyed attempt to dredge up some superfluous lore from the mobile game I reject with my whole soul at the expense of a character with awesome potential. What is effectively the difference between a Darkling and Heartless? My guess is, very little, but slap a fresh layer of paint over common terminology and I guess anything is recyclable.
I've pretty much given up on the mobile game except some of it's core ideas. Keyblade wielders who have fallen turn into those things. They never stated if there is a difference between a Darkling and Heartless. There probably isn't a difference at all other than it's a purebred heartless rather than an emblem. These Darklings could be the very first heartless to truly exist, that's my belief. All the heartless you fight in KHUX are fake illusions except for the Darklings if I recall correctly. Those things can even speak.
I mean, all those purebred heartless were human/normal once, weren't they? And we know killing them, even with the keyblade, doesn't release their hearts, right? I'm just too curious to know if this is the true method of making a purebred heartless, a being covered in and distorted with darkness. It's not really NECCESSARY to know what makes a pureblood heartless compared to an emblem one, but if Aqua is showing the signs of the natural progression of becoming a pureblood heartless and that's how it's always been, then I'm for seeing it play out. If that is the way the universe works, Aqua shouldn't be an exception no matter how much we like her and we don't yet know how this will turn out for her characterization or her future.