The final season fucked up one of the core pillars that made AOT that one special anime, beyond everything else. Before, the story and the world-building felt completely justified, even the fantasy aspects, because causation begets consequence. However, the final season broke that. New plot elements were introduced far too many times without a proper long-term basis to them. There was no smooth buildup and no logical backup except “trust me, bro.” The reason why I loved AOT so much in the beginning was because it made a point of not doing exactly that. Although it is pretty common in normal shows, I grew not to expect that from this. So I wouldn’t say that this is a perfect final season, but it sure as hell was fun to watch.
======== Many people complain that Hallucinogenia is a bad conclusion to the Titan mystery, but I personally think it could not be better. First of all, it’s not some made up bullshit but an actual worm we know nothing about - so it might as well be some supernatural creature that died without having a shot at turning someone into the first titan, and on some parallel universe, Attack on Titan could’ve actually happened. This makes it fit perfectly with the realistic and grounded nature of Attack on Titan’s plot. If the solution to the mystery was magic or aliens, like many fans speculated, this would’ve been just lazy and unimaginative. This is perfect. Secondly, hallucinogenia fits perfectly with how Titans actually work. Titan powers come from inside the nape/spine of the shifter and from a mysterious elecrical charge. So it’s only fitting to Hallucinogenia, who literally looks like a severed human spine, through which our nerves run electrical impulses. Thirdly, with Hallucinogenia we finally have an answer to what the world of the paths is and how are all Eldians connected. If Hallucinogenia attached itself to Ymir so it would be able to multiply, it only makes sense why Eldians all carry Founding Titans/Ymir’s genes. Hallucinogenia multiplies like a sexually transmitted disease, its cells are spread out into every single Eldian from birth but are not awakened unless they are stimulated with a spinal fluid of a titan, or, in other words, already awakened Hallucinogenia cells. The reason why only 9 titan shifters are able to willingly control the state they are in, is because 3 daughters of Ymir did not merely inherited non awakened Hallucinogenia cells from birth, but actually ATE hallucinogenia worm itself which was attached to Ymir’s spine. Later their children also ate their spines, because that’s what king Fritz demanded, and so original Hallucinogenia worm, which is literally like a hive mother to all Eldians who possess its spread out cells, was spread evenly between 9 individuals, and is passed throughout generations by eating. The founding titan is simply the Eldian which possesses the biggest piece, or the head of Hallucinogenia. This is how Eren had it in its spine when he and Zeke made contact, it was there all along and in Griesha/royal family before that. And the hive mother Hallucinoenia - the founding titan is what connects all its cells and titan shifters, just like a bee hive mother is connected to all the bees. And when an Eldian dies, Hallucinogenia collects back its multiplied cells, and since they are situated in the spinal cord of a human, it collects conciousness and memories of those they belonged to. This is how Eldians end up together in the world of the paths, which is simply the nervous system of the original Hallucinogenia.
The ovas technically support the different timeline theory. Technically he says that this world, or potentially the timeloop exists because of Mikasas inability to accept erens death. Every single time weve seen the story “end” is a timeline where Eren dies and Mikasa griefs it and then a new universe happens. If the loop exists and the theory is true, then the anime only ending would be the hypothetical timeline where either Mikasa does accept the death of eren, or Eren doesnt die therefore the time doesnt reloop. It would be interesting to see if Ymir was the one attached to Mikasa because she also was stuck in a loop of being loyal to the royal bloods, so she was using her powers to try to give Mikasa, someone with the same attachment as her, an opportunity to live out her attachment or something like that.