Gotta go fast
I'll give my suggestion for where things went wrong. There was no plan beyond finding out that there are more people outside the walls, and Isayama was easily influenced and would look for outside reception more and more as he wrote the story.Having now read the OP and the highlights, all I can ask is how? How can something lose its way this badly? What happened to the AOT of about the first 50 chapters or so? Was this manga always doomed to go to shit when Grisha revealed that humanity existed beyond the walls once Eren reached the basement?
As I was going through the thread, I came up with an idea: have Paradis be like old Australia and just have it filled with prisoners and exiles and then just dump a bunch of Titans in there and we have the setup of the first arc of wanting to exterminate these Frankenstein’s Monsters of cannibals. And then just have it where the Titans are made to just be mindless to keep the island people from getting too uppity or asking too many questions that hit too close or make any investigations. And instead of Marley vs Paradis, it’s just a way to overthrow the regime that’s behind the Titan program and the likes of Eren and Mikasa and Levi and Hanji and Connie and Sasha can live in some kind of peace. And the Titan thing was probably some generic mad scientist shit, or Josef Mengele if you wanna go Nazi with it.
Probably has the same pitfalls and problems with genre shift as main canon, but I’d like to think this idea is simpler and doesn’t have to deal with pigs, Ymir’s views on love, paths, mind control, or whatever that kept Eren from getting into Mikasa’s pants.
It also seems that AOT looks to be at least hilariously bad. That’s the best kind of bad.
Like let's look at world building. If the plan was always to include Marley, the Tyburs, Helos, etc. I think there would, or at least should be, less vague ideas about what is out there. There was clearly some change in idea as there's no hints of internment camps before the basement reveal. Instead Annie is shown to be living in the forst, and Ymir came from a backwater village. So Isayama always planned for there to be humans outside the walls, but had no solid idea of what the world actually looked like or why the Warriors were trying to kill the people of Paradis.
Other issues with the world building is the lack of exploration of the world inside the walls. It's so vague we don't even know the names of the political leaders who get overthrown during the Uprising arc. I've seen a lot of people say that this is the first misstep in the story, and I think it's because it's a relatively simple coup against a few nameless leaders.
If the men in power behind the scenes had already been introduced around the time that Eren gets his titan powers then perhaps people would have been more invested. Create already in the early arcs the idea that there's a conspiracy within the walls to maintain peace and order at all costs.
Once the Marley arc begins, I don't actually mind the timeskip initially, and I can see why Isayama considers it the time where he had the most fun. The Marley arc actually has some solid world building in it, and actually makes a lot of effort to try and mend for the fact that there was clearly no goal for where the story was supposed to go after the basement reveal. The problem comes from the realization that the timeskip happens too near the end. Pacing wise Isayama is introducing a dozen new characters who are supposed to have their own motivations and characters arcs, but because there's just two arcs left they remain relatively static and a lot just ends up being wasted. I get that after a decade you want the story to end, but this again shows the lack of longterm planning. If you know you're going to have to both introduce a whole slew of new faces, while also explain what all the old characters have been up to for the last four years, you're going to faces issues, and there seems to be consensus that a lot of characters, both old and new suffer from a lack of direction. Take Mikasa and Armin for example, they're supposed to be part of the main trio, and yet for the last two chapters they're pretty much just along for the ride and have little to no goal until their mission becomes to stop the rumbling once it starts.
If the rumbling and the Paradis civil war had been their own arcs, then perhaps the author would have had more time to create actual motivations for everyone, and for the civil war to actually be a civil war, and not just Floch leading a one man coup while Eren mopes around and Mikasa and Armin are dragged from location to location.
Then there's Isayama admitting he started reading more and more fan responses once the story neared the end. This was likely a bad move, and again shows he was unsure of where to go. I don't think I've ever seen a story that got improved because the author started listening to the fans.