I mean, I think we've done tons of treatments through the thread, may as well do mine now. I'll do a challenge and keep as close as possible to what Isayama wrote....for a little bit.
So you keep most of the stuff from the first third, including the mysterious warrior bit. Things are basically the same up until Annie, until you have some mysterious figures pressuring the government to stop the Scouts. I'd have more characters, but basically the military police start executing members of the scouts. This leads to the Overthrow arc with the royal family. Kenny's an assassin who has met with these mysterious figures, but instead of being brainwashed, he only desires their power. It'd be at this point, I'd slowly reveal that these 'figures' can create and control Titans. We already know Titans can transform at this point, but now we learn Titans are humans.
I'd skip the titan formula, we're not going through the dilemma. So, shit gets overthrown, Reiss goes nuts, Historia crowned. Now they go to take back Shigishina. Colossal Titan gets wrecked by Armin, Armin lives. Levi gets rock smashed instead of Erwin, because fuck Levi's perfection. So either dead or out of the scouts. He needs to be nerfed. Erwin lives. They go to the basement. Turns out its cyberpunk apocalypse.
The Titans are humans, and the age of technology came about during a self-inflicted cataclysm. Titans are bio weapons, created by former humans known as 'the Pure'. The warrior titans and their society outside the walls are enslaved by the technologically advanced 'Pure'. Eventually its revealed that the 'Pure' adopted eugenics wholesale, the problem is that because of eugenics and their 'perfect' immune systems, there was no randomness, which is necessary for an adaptive immune system and most of them are wracked by innumerable diseases which evolved against the Pure who are only alive due to their technology. Due to their arrogance and need for perfection, they have long since destroyed 'imperfect' stocks and need fresh genetics. The Eldians and the Warrior society are the last real humans as the 'pure' can no longer interbreed with them and look like lepers. The Titans 'eat' humans because they're brought back to the Pure for experimentation and not all of them are killed when devoured (double dose of horror). Levi and Mikasa are so good because some of the Pure defected to Eldia when they could still breed with humans. So there are random super-soldiers scattered about Eldia, which pisses the Pure off more since they're basically the ultimate in humanity.
So its on the scouts to convince the warrior caste to join them, and fight against an army of Titans and eradicate the 'Pure' so they could be free of Titans and these monsters. You could introduce body horror elements, ruins of ancient civilizations, keep the titans, and all that shit.
As to who Grisha is, he's from the same bloodline that defected from the Pure centuries ago and the knowledge was passed down from generation to generation. But there's two sides: defectors from the Pure who hate them and defectors from the Pure who idolize them. Those who idolize them are in the government, along with King Cuck, who passes down the power of the Attack Titan, the most powerful bioweapon, to his bloodline. Grisha does the shit he does in the manga, but is mortally wounded and forced to pass down the Titan to Eren. We'll skip Zeke as being Eren's brother and that whole story, he's a member of the warrior caste who is a loyalist and believes the Eldians should be peacefully sterilized into oblivion. So the conflict between the descendants of the Pure would be the royal overthrow arc, which you'd learn about later. No time travel, no fucking Gods, none of that shit. Man vs. Man, Man vs. Nature.
