If you try to take OG SAO characters and force a realistic NTR out of them, it’s never going to work.
It will work easily, but someone is too lazy to come up with something better than a patchwork quilt of other people's ideas, characters, images and techniques. It's not so much fun anymore, because you'll have to really think about doing everything well and the money you earn won't be as easy as it used to be.
Here's an idea that I came up with, literally, now - the whole FOG game - is the creation of Sugo, who couldn't get Asuna during the events of Fairy Dance and decided to play a long game. After leaving some of his influence, he decided to contact his allies so that they would create a game with the necessary assets at its core.
Kirito and ASuna start playing a new game, and during the story quest, the boss monster leaves a "Seal of Change" on Asuna's body. At first, Asuna and Kirito don't understand what it is, but they find an NPC who says that a long quest chain is needed to remove this seal(can add a moment that Kirito finds out on the forum that no other player has seen this quest).
Naturally, this is a trap created to delay the time before Asuna's mind is changed. Over time, Asuna realizes that she has strange desires inside the game, which grow every day, and even Kirito cannot fulfill them completely.(it's not necessary to make Kirito a small dick, it's enough to make Asuna just very lustful and insatiable). One day, Asuna went alone on a quest to collect resources and due to the activation of the seal, she almost lost consciousness and in this state Inoda finds her, Asuna's lust is activated and she fucks with him. When Asuna woke up and realized what she had done, she didn't have the strength to ruin her long-term relationship with Kirito by getting married and going to the United States. Inoda sees the seal and understands what is happening (since he is a child who has played many porn games), so he offers Asuna help. Asuna initially refuses, but eventually realizes that she can't handle it on her own, and Kirito is already beginning to suspect something because of Asuna's boundless lust at night. In the end, she agrees to start small in the game (followed by slow corruption).
A little later, Asuna realizes that the seal affects the real body... Then you can come up with it yourself. It's not difficult.
It's not necessary to make Kirito stupid, because Asuna is also a smart, grown-up girl, and if she wants to hide everything from Kirito, then she uses her mind for this, and not just hoping for Kirito's brainlessness. It's not necessary to make Kirito's cock small, as I've already written. It's not necessary to make Inoda a manipulator gigachade, because Asuna practically fell on his head and agreed to his help. You can even make him more stupid and weak, for a contrast with Kirito.
And we don't need these leaky and complicated plot with an Administrator, a Council, or some kind of spies in the form of Sinon and Agil. Alice and Klein are not needed, so as not to overload the narrative. Can add a storyline in which Kirito is busy protecting Suguha from Hebeshima (which is why he may be so inattentive to what is happening with Asuna).
In the form of a global plot, the detective reports that Sugo is doing strange things in prison, and Kirito learns through his channels that FOG is somehow connected to Sugo and Aincrad. And begins an investigation. That's all. A simple logical plot, well-embedded characters from the original SAO, slow corruption. If I wanted to take the time, I would have written this story for 20 updates, but I'm too lazy. Ha ha!
I'll say it again, Fujino just wanted to do it as usual, but he didn't take into account that there is a huge difference between an instacorruption without a plot and a story game with slow alignment. He tried to do everything the same way he had done before, on a lazy vibe, but, in the end, he took a shit.