The scenes in the extended ending are really my only complaint about the entirety of Aquaria - I'd just like to lay that down to start off with so no one thinks I'm ungrateful or don't appreciate Aquaria itself - it was overall an amazing game and if all I can complain about is a part of the ending that's something impressive...still, that said, let me begin with my complaints.
Mostly, my complaints are that the entire series of events seem clearly like they were written just to open the game up for a sequel, and so they are rather forced and not as well written, especially in comparison to the masterwork that was the rest of the writing in the game. This is a problem even mainstream, professional companies fall victim to - god knows Bioware forces their writing all the time in Mass Effect 2, because they want certain events to happen, they will make characters do things even if it's not entirely natural, so it's understandable that the same could happen to Aquaria, but all the same it's a flaw. Not to mention, the entire extra ending serves just to be a downer ending so that there is motivation for conflict in a sequel - but since it is very unlikely there is going to be a sequel, it really feels rather pointless. The ending was pretty much nice and self contained before the extended scenes which just kindof ruined the tone and emotion for something extremely depressing, all in the name of a game that will never be...
So, that's all a general idea, what do I specifically mean, when I say things are forced? Well, it's just...general inconsistencies and problems. For one thing, where is Li in the entirety of the ending? Naija gets kidnapped by her mother and he doesn't do anything about it? He's just gone? After all they've been through and done, stopping an entire god, a crystal is what stops Naija?
Which is, really, another problem that's forced. Naija needed to be incapacitated for a sequel so as to provide conflict and motivation for the player to care, and also to give reason for there to be another protagonist, so she just became unavailable, even if it doesn't make that much sense that she would be. So why do I find the way events turn out so unreasonable? Well...it was a rather obvious trap, I think, and it's a little immersion breaking that Naija, who has been through so much, can be dumb enough to fall victim to it without even resisting.
It wasn't even a surprise attack, which is part of the problem. She goes through a very dreary looking passageway that wasn't there before, full of dead things, and meets her mother. If her mother had just instantly taken this moment of shock to incapacitate Naija it'd be one thing, but she didn't. She went on a monologue detailing just how evil she was and how little she cared for Naija, even specifically stating how she'd harmed Naija in the past.
So the problem? None of this puts Naija on her guard. She doesn't resist her mother more than slightly verbally - she should've been expecting something from the moment her mother said she wiped her memories in the first place. She tells her mother to let her go, but she doesn't resist at all, she just lets her mother do with her as she wishes. This is what I mean by forced - Naija has, throughout the entire game, been a rather strong, independent character who would fight for want she wants, and here she is entirely passive. It's inconsistent!
At the very least, there should've been an unwinnable boss fight that shows Naija simply being overcome - that would be more consistent. Naija fights, like she always has, but for a change simply is overcome. Considering she was able to take down a god, not being able to take down Mia is somewhat hard to believe, but then again she had Li with her against the Creator and she wouldn't have any emotions holding her back, really, like could be present against Mia - and before anyone says it, I don't think any of these emotions should've been strong enough in acting against her to explain why she is so entirely passive in the confrontation with Mia. She was strong enough to overcome emotions to do what needed to be done before, now all of a sudden she isn't? That's why I don't think that fits as an argument to explain her being passive.
Anyway, that's really my issue. I am, of course, just bitter than an overall really well self contained game had to be forced into an incredibly depressing downer ending that will never get resolved, but I do think I have some valid complaints against the extended ending, as it was, anyway. But I do think I would've honestly preferred if the game had just ended without the extended scenes - I really kindof regret getting all the memories.
Edit - To anyone who may have agreed with me, Alphasoldier has a great theory that solves many of these conflicts on page 2. Read it!