I haven't seen a thread about this one... and figured I'd post some thoughts on it so far (about to hit the final fight), with spoiler tags as appropriate.
In Short: The early section is great. The later section feels rushed and (disappointingly) pulls a switcheroo on theme.
Suggesting a game that my wife & I have tag-teamed up to 75 hours of play time (and had over 10 years of development) feels rushed might sound strange, but 60+ hours of that was spent in Lucis, cruisin' with the bros, hunting monsters, racing chocobos and, yes, fishing.
The early section of the game has a strong and frequently reinforced theme of unity, brotherhood and "together we shall triumph". It also features constant banter (and bad puns) between the four characters, establishing them and their relations to each other, while you go through the immense amount of available content and activities. There are some patches of weirdness (Gladdy leaving your party to "do stuff for a bit" with no explanation of what and why ever coming up again in the game), but on the whole it is excellent and will keep you entertained for a very, very long time.
Exposition on the main plot comes at a comfortable pace that neither takes too much of a back seat, nor hijacks control of the game to run you down the railroad tracks.
If the rest of the game maintained this theme of brotherhood and unity, then I would heartily recommend the game.
And then you leave Lucis (), and continue with the main story. And quickly and suddenly switches theme and tone to "Noctis' sacrifice for the greater good" and "Dark and Foreboding".
From here on, you're on railroad tracks as the game rushes you through a series of somewhat arbitrary and inexplicable plot points to dump you into a frustratingly long and unnecessary dungeon, and then the final sequence. Most of which is arbitrary, brutal and riddled with plot holes even Prompto could drive the Regalia through.
As someone who was enjoying the game and gradual build up, having the sudden shift to be a rushed (and at times confusing) tour of misery was bitterly disappointing.
Some key points:
The Leap-Frog in Time: At the end of the dungeon-from-hell, and just before the last fight sequence, the game leaps forward 10 years in time. Ten years. All of which was spent in darkness (though there are still plants), and most of the world's population is dead. Yay?
This is arbitrary and largely pointless, and the game gives a bit of exposition on what everyone is currently up to, and lets you prepare before you go on to the last fight... or back in time via your magical dog. It also switches Noctis' model to an 'older' Caucasian form... which looks nothing like his younger (and decidedly Japanese) version.
Blinding Ignis: So shortly after leaving Lucis, Ignis gets blinded. Permanently. The fact that this is happens (and leads to some incredibly frustrating gameplay) is irritating, and the fact that it happens entirely off camera is unnecessary and arbitrary. You know those abilities you leveled up of his? Gone; His abilities are changed (and generally crap). Indeed, for a large portion of the remaining game the various recipes and ingredients you've collected for awesome buffs... aren't available, either because he's blind and can't cook, or he isn't with you.
Killing Lunafreya: So Lunafreya is Noctis' childhood friend and now fiance' and the Oracle (never properly explained) who can heal people of the Starscourge (implied that she draws in the sickness out of people and into herself). Cool. She is also awakening the six gods so that Noctis can receive their blessing and fulfil his destiny to become the King of Light etc etc.
Cool.
So we finally get Noctis to be in the same place as her, share a nod, and go off to face Leviathan (God number 3). And then she gets murdered (potion, anyone?). Noctis literally didn't get to say one word in person, and she becomes a woman-in-the-refrigerator to motivate him to killing the BBEG. We had received some exposition on who she was via various flashbacks, but ultimately there was a character there we should have seen more of... but instead she dies the moment we meet her, and before her supposed role in the plot is complete.
Dead Gods: There are six gods in this world, and Noctis is supposed to get the blessings of each. Enigmatic, but sure. Except the empire is also killing them... which doesn't appear in any way to impede Noctis' ability to summon them to smash his foes. Weird... but okay, whatever. The gods are also not all friendly: Leviathan in fact threatens to kill every last human she can reach if Noctis can't beat her. Charming.
Titan (god 1), you fight a bit and flee. And then he dies off camera.
Ramuh (god 2), you never see in person, but get the blessing of by a treasure-hunt quest. And then he dies off camera.
Leviathan (god 3), you go through an epic boss fight with and kill yourself.
Shiva (god 4), gets killed off camera by the empire before you get anywhere near her. But also doesn't. And you get her blessing by her corpse... okay?
Bahamut (god 5)... lurks out in space and is enigmatic as hell. So who knows?
Ifrit (god 6) is the penultimate boss and a bad, bad dude. Who you never see until the end sequence, and is not the final battle... Ifrit's minion is. Sense? None.
Noctis Must Die: So rather than a theme of unity, the end sequence changes to the theme of martyrdom. Sorry, let me correct: Arbitrary martyrdom.
Noctis must defeat the BBEG and restore light to the world (noting the game had it arbitrarily be in darkness for ten years), but doing so will cost Noctis his life. Because.
But what about your buddies? Well, apparently they're not actually important enough to even warrant a bit of a cutscene or exposition on what happened to them after the game ends. Seriously? I just spent 60+ hours with Prompto, Gladio and Ignis, I want to know what happened to these guys.
Locked Content: A chunk of the game is locked until you finish the main plot, wherein you go back in time and can play more of the game... after it has thoroughly beaten any desire to do so out of you.
This was horrible in FF13, and is even worse in FFXV. You're done. The game has ended. There is no motivation to go and do extra dungeons because it doesn't progress towards anything. Game Over man, Game Over.
I'm aware they got antsy about people playing their optional content to the point of trivialising the main plot bosses in FFX (I mugged the endboss to death in one hit), but that is a problem with having really bad boss design and going overboard on what the optional content can give, not an issue with having optional content at all. FF7 and FF8 didn't have this issue, as Sephiroth and Ultimecia were sufficiently uber that even completionists still had a bit of challenge at the end, while still feeling that all that effort was worthwhile.
The Big Bad: So the big bad evil guy is Ardyn, formerly the one chosen by the Crystal (minimal exposition in game on that either) to fight the first Starscourge, which Ifrit made because he's a jerk. Okay. Ardyn instead decided to heal people by drawing the demons/plague into himself to save thousands of lives... a bit like Lunafreya does.
Okay.
But apparently, the gods (who like to be enigmatic and give very little instruction/advice) didn't like this and made someone else king instead. And Ardyn was made into an immortal by the demons inside him (but Lunafreya and the other oracles were not?). And now Ardyn serves Ifrit (he of daemonmakingness)... but is the final boss of the game.
And his motivation is that he wants the world to fall to darkness and Noctis to become the "King of Light" so he can kill you and feel better about himself?
Square Enix, you are strange.
In Summary: Square-Enix couldn't seem to make up their mind about what kind of game they wanted to make, and so released something that was two different concepts stapled together.... and it doesn't work that well.
A game about unity, brotherhood and four friends saving the world would have worked.
A game about grimdark self-sacrifice would have worked.
A game that is 75% one, 25% the other will please very few people.
Yeah, honestly this is likely my last FF game.
I might not have liked the play of 12 or the Playboy bunnies for no reason but the plot was alright and the world development worked for me.
13 was a trainwreck I still haven't managed to force myself to finish.
I can't even care about 14 MMO.
And then there was this.
I'm past Altissia, which is the theme change you mention. It felt so jarring and pointless that I'm not sure if I'll load the game back up. Having you fight a god should feel epic, but having you automatically win is the opposite of epic. It's boring. It could have been neat if the fight was actually a culmination of having honed your skills with Noctis, but instead it's just a button mash fest, where what you press doesn't matter.
Then Gladio's dialogue on the train was so bad I couldn't pay attention any more. I had to shut it off.
Ifrit's motivations are cool atleast.
Yeeeeeahh.... No.
I'll be honest, I didn't get that far through the Pitios Theory before I had to walk away. It reads way, way too much like the ME3 Indoctrination Theory posts. Only much worse.
Arguing that there is some kind of metaplot in the background based on features in random dungeons that has "Greater Meaning" when put together that somehow validates the game despite its failings is... Wishful thinking in the extreme. Worse, the PT is basically suggesting that the player's, and Noctis' journey is basically a pantomime and devalues it even further.
The story isn't that of Ifrit. It's of the bros, Lunafreya (to a minor degree, sadly), and Arden. If they wanted to tell a tale of Ifrit trying to save the world by rescuing trapped gods or whatnot, they really, really missed the mark.
I didn't mean to imply that the metaplot explained in the bonus dungeon ties everything together, I just thought it was a interesting look into Ifrit and WHY he out of all the espers are opposing you. 'sides leviathan whose a dick.
That's why it's a bonus dungeon. And honestly it's a bit too symbolic to just wave it off as an Indoctrination theory bull which was based primarily on the fact that people didnt like the ME3 ending.