Considering I've already written over 7,000 words on the original Tsui no Sora on this site, I intend to keep things a little shorter for now. My intentions below are to both discuss some route-by-route feelings in comparison to the original game, as well as further ruminate on the philosophical and thematic elements of both Tsui no Sora and Subarashiki Hibi. I'll give a spoiler warning when we jump full-on into dangerously heavy spoiler territory below. Still, mild spoilers for all three games within.

I don't think you need to have played the original game to appreciate the remake, you certainly need Subahibi, though. The original is helpful for understanding some of the deliberate deviations the remake gets at, some of which are pretty major, but you could feasibly play this with just Subahibi under your belt and get 85% of the point. It's hard to image what sort of impression the remake would land if not taken as an addition to the previous iterations of this story. I think above all else, SCA-Ji was able to more directly address additional influences over the original Tsui no Sora in the remake. Not only did he buckle down and more directly address the most obvious influence, being Nietzsche and particularly Zarathustra, but the influences of Spinoza and Lovecraft are even more clearly dug into and elaborated upon here in ways that are rewarding if you've spent time thinking about Subahibi for as long as many of us have.

Presentation-wise, the remake is a very interesting hodgepodge of KeroMakura styles and talents. Motoyon and SCA-Ji both take lead artistic roles here, and while SCA-Ji very much still draws in the same style he has since around the H2O and Subahibi days, Motoyon was clearly already adjust his style for the upcoming Sakura no Toki and so it comes off much more modernized. Motoyon also did most of the CGs, which are absolutely beautiful. Rarely as abstract as some of Subahibi's, but beautiful nontheless.
This sort of disorientation is largely supplemented by the fact that many background elements are ripped right from Subahibi, and the soundtrack is pretty much all the original ~15 second loops from the original Tsui no Sora. ... Until this isn't the case from Fourth View onwards, where elements of Subahibi begin to melt in from the soundtrack to a few pieces of CGs. The voice cast is largely different, and the only people who reprise their roles from Subarashiki Hibi do so with intent. Largely great performances all around. Each of the ending themes is an instrumental remix of one of the Tsui no Sora tracks and they're all fantastic. I think my favorites were Third View and Numinöse II.

On a route by route basis -

First View was a bit different from the original, largely in how it characterizes Yukito. I really liked the more grounded and empathetic view of his character this time around, and I wouldn't say it "replaces" the original because they're largely coming from places with different intentions. Otherwise, note for note, extremely faithful to the original everywhere it should've been. Second View was definitely interesting. There were a few moments of characterization missing that I loved in the original, even if they were small beats, and at first I wasn't sure how I felt about it. But once the plot started ramping up and the big deviations from the original plotline kicked in, largely revolving around Yasuko's feelings towards Kotomi I ended up absolutely loving it. Despite some minor nitpicks in the first half it's absolutely a stronger version of this story line. I have little in the way of notes for Third Route. It's just a straight up improvement over the original in every way. Admittedly cried to the ending.

... And that's where things got really interesting.

Massive spoilers for the remake and Subarashiki Hibi below.

I had heard tell that Yasuko of all characters was the protagonist of a new route in the remake, but somehow I wasn't expecting it to be earlier in the game than Takuji. I also wasn't expecting it to be arguably the best part of the game. She's simply one of SCA-Ji's best characters I've yet to experience. A deeply morally conflicting character who scratches directly at the wound that plagues every iteration of this story - the incessant babbling about existentialism and purposelessness of life means nothing in the face of love. And Yasuko is a character who has had her humanity stripped from her by force - but still clings to the subconscious believe that love is worth fighting for. You can say she's really fucked up for how she tries to get with Kotomi and you're definitely right, but by the end of the route, when she and Kiyoshi just get to have that heart to heart... shit, man, that's the point of the whole story. That's what each version of this game has been trying to say since 1999. Even if SCA-Ji himself couldn't have put it into words yet.

Final View was certainly interesting. I really loved that some of the abstract visions from the original game were left intact, 1999 artwork against 2010 backgrounds and all. I also really enjoyed how much Takuji and Riruru were given time to expand upon their relationship to one another. One of the strongest elements of the remake's stab at this story is how much it points its cast in the direction of "reality of the heart". In the same way that Yasuko had to come to terms with her feelings for Kotomi, Takuji has to do the same for Yukito - the nature of those feelings may be vague, but I chose to interpret it as an attraction of the mind. Maybe there's some homo-eroticism in there and I wouldn't deny that, but I think it's more that Takuji sees a home and a place of understanding in Yukito. I'm... gonna have to sit with the moment he hears Kimika's voice and unbox why that was for a minute. It's the most confounding and jaw-dropping moment in the entire route and it's gone within a second. Again, there are a few minor things I do miss from the original, especially some of the more abstract denpa effects like Riruru's number spiral form or the eyeball bleeding into the pool, but this version of Final View has many aspects that undeniably surpass the original and at the very least do the series as a whole serious justice.

And then there's the Numinöse duology. Numinöse I seems largely unchanged from the original, but that's fine - I already like that ending more than most. Numinöse II has some interesting changes itself, and the duology as a whole stands as a rather interesting conclusion to the series overall. I'm happy that SCA-Ji keeps it vague and never actually explains what Ayana is. The Nyarlathotep explanation is an interesting proposition, the Cthulhu explanation is also interesting, and the way she molds and shifts around her necessities and whims, or those of others... all interesting. As far as where I stand on Ayana, and the purpose of the End Sky itself? Well...

First off, it was incredibly gratifying to get SCA-Ji backing me up on my original read of what the End Sky represents - it's the doorway to an Übermenschian evolution, the discarding of humanity and subjectivity to become, as Riruru and Ayana put it, a "third-dimensional existence". I think this is the purpose of Yasuko saying what she does on the rooftop to Takuji - to discard humanity is to discard our ability to love, because love is founded in perception and irrational thought. It's not so much that the End Sky exists, or that it doesn't - it appears in many ways to many people, as an escape, as an evolution, as an ending. Perhaps that's why Kimika's voice reaches Takuji in the end, as a reminder that he is human, and is capable of a love that transcends his own delusions. I've said it before, and I'll say it again - Down the Rabbit-Hole I and the "good endings" of Subahibi are the entire point. They're the response SCA-Ji makes to himself and his work in Tsui no Sora, and now he's returned to this early work and planted seeds of that answer into the bleak and hopeless place he left behind in 1999. I do believe the only presence that actually does sit above humanity in all of this is Ayana - who is more of an idea than a tangible thing. I think you can rationalize that existence many ways: a monster, a schoolgirl, a cosmic terror, a god, a magical girl - but ultimately, Ayana is objectivity. She is the third-dimensional existence that plays out stories and creates scenarios for the purpose of understanding humanity. And if it's not directly her doing - though clarity on that isn't really necessary - then she's merely observing. It's not that she's cold, it's not that she's cruel, feelings just don't process for her. It's all an imitation or an attempt to emulate expression. But that begs the question, which she asks Yasuko - what's the difference in attempting to understand and engage with human emotion and actually having those emotions, considering neither exist on the plane of logical thinking or rationality either way? Where does the line blur between emulation and experience? Fantasy and reality? Truth or feelings? An ending or a beginning? Takuji or Yukito? Ayana or Kotomi? The End Sky. The unreachable all-knowingness that we seek in spite of its non-existence. Human ambition. Yearning to understand. Indulgence in sophistry. The answer was already in front of us. Live happily in the wonderful everyday.

SCA-Ji is the fucking man. That's the be-all-end-all of this whole thing.