I was currently watching AoT and I knew things would fall apart catastrophically at some point, now I know what. I was eager to find out just what the hell is going on in that basement and ... . Okay. That's really exactly the way how not to do it. Any reveal in the basement should have expanded the setting with the Titans, not replace it like this. I mean, the "it's just Titan-Auschwitz" might be a somewhat interesting turn of events, but when it completely replaces the storyline and setting that people enjoyed in the past, it's not hard to see why people are upset.I'll never fucking forgive that cunt for that. He started watching Game of Thrones and then was like, "ME WANT" and just did that. I don't think its an exaggeration that the manga lost basically half its audience after he made the switch and the basement was revealed. I seriously think the only thing that actually saved the manga was the Anime, and the Anime is basically relying on the first part that everyone loved, I have a feeling its going to crash. And crash fucking hard. The only people I see really left are shippers (oh my fucking christ, the shippers in that community), people who think he's a secret genius and fanboys who don't even think, just consume.
The funny thing is, fanboys are like "OH GOD, WAIT TILL THE ANIME SEES THE BASEMENT REVEAL LOL" and I'm like, "You mean when he upsets everyone for making his universe a worse game of thrones and loses half his audience because he basically threw out what they loved? And then he does a time-skip because he shifted the entire genre and had no fucking idea what he was doing and has been winging it ever since, and character motivations haven't made sense for years with 0 revelation because Isayama doesn't actually know what he's doing?" I'm really going to love when the brigade rides out to defend that fucking shit show.
I mean, for non-anime people, I really feel like I have to explain how really jarring this was. Attack on Titan was this post-apocalypse setting where the remnants of humanity were set up in these huge walls, surrounded by gigantic titans, monsters that would just eat people. Then you gradually learn there are sentient titans, humans that can control these monsters. So there's this big mystery, the primary genre is the horror of people fighting these things. The answers are held in the protagonist's basement. For which there's this huge charge, a major sacrifice, huge character deaths to get to.
Then it turns out that no, humanity is thriving and just fine, and the island is just a World War 2 concentration camp (figuratively) because the world hates the
Trump's Chosen Peoplepeople on this island and they're there for punishment. They just have a deus ex machinamagic titan that uses the MIB mind eraser on the entire population of this island to forget the existence of the rest of the world to make an excuse as to 'why don't they know the world exists'. Then there's a whole bunch of bullshit nonsense with royal families and bloodlines and shit and eventually most of the 'fighting these horrifying titans' are replaced with some stupid fucking world political machinations and its just fucking nauseatingly bad, where Isayama did a major timeskip after this revelation and basically the protagonist's motivations have made absolutely no sense since then, because he clearly doesn't know what the fuck he's doing. Fuck even the major ANTAGONIST'S actions make no sense at all. Even WITH what he revealed.
2 other examples come to mind from Manga/Anime: Claymore and Vinland Saga.
In Claymore, we have a medieval setting with shapeshifting monsters (called Youma) that hide amongst humans and eat them. Humanity can't match their strength on its own, so they create half-youma half-human hybrids called "Claymore". Said Claymores carry huge swords, can unleash their Youma-powers and fight... however there's some shady organisation involved. Essentially, a dude will enter a village with a Youma, offer to send a Claymore to said village and once the village has been cleansed, he comes again to pick up the payment.
In the later chapters, so called "Awakened Beings" emerge, what at first looks like incredibly strong Youma turns out to be Claymores that unleashed too much power and transformed into monsters even worse than Youma, insanely strong and insatiable hunger for human guts.
Where this becomes incredibly similar to AoT is that in some chapter, it turns out that the entire continent that this is settled on... is a weapons test facility by the guys that operate the shady organisation and they want to make the perfect weapon (either Claymores or Awakened Beings) in their war on the mainland.
I don't know if the author got backlash, but the later chapters of the manga pretty much ignore that whole point and the story goes back to where it should go. For a time, I was afraid that it would turn into "Main Character goes to the mainland to fight in the war/against the assholes that keep an entire continent as a weapons test range", but that thankfully never happened.
A reveal like that doesn't feel like a big expansion on the setting, even though it adds fluff, it feels like it takes away from the setting.
Claymore narrowly escaped the fate of AoT, it seems.
The other example, Vinland Saga, is pretty interesting, too. There's 50ish chapters of vikings doing viking things, with some intrigue, backstabbing, betrayal, etc. and a lot of fighting.
Around the 54 chapter mark, shit goes south in a very major (and absolutely amazing) way for one of the more important characters, the main character is taken prisoner and ends up as a slave. He then vows to never kill someone again and the whole story shifts tremendeously. It was jokingly called Farmland Saga, since Thorfinn (the MC), spends a few chapters working as a farm slave, until he escapes. The whole thing still has some action-scenes, but not to the same extend as it used to
Many people didn't like the way how it shifted away from gore-y war action, but the story continues strong and it feels like a logical developement of the story and characters. I personally liked the change of pace and I think it didn't suffer a cataclysmic exodus of fans and is still going strong.
So, there you have it. Three instances of a story changing element being introduced: 1 falling apart, 1 having a near miss and 1 actually being successful.