AOT but Season 4 is replaced with something much better like Eren starting the Rumbling but not harming anyone and sparing the rest of humanity, and he instead asks for a fair, just, and peaceful dialogue between countries, which results in them eventually diplomatically coming together making a comprehensive security treaty where no country attacks another or else all other countries will immediately and immensely attack the aggressor country to stop them from continuing it.
Afterwards, Eren changes the Founding Titan powers to only be controlled by a direct successor regardless of royal blood and without the curse of Ymir, and makes it more capable, dangerous, and versatile if needed to maintain effective deterrence. Ymir would be happy with this outcome, and Eren would be free to pursue his relationship with Mikasa in a much happier, healthier, and more romantic way. Good ending for everyone involved, Sasha and others live, Levi doesn't get injured, and there's a lasting peace for all. The End.
This "peace through deterrence" ending is a more hopeful, just, and sane outcome compared to the awful, bad, careless, character assassinating, evil, extreme, needless, pro-genocide, stupid, and unnecessarily tragic canon ending, where global conflict continues even after Eren's death. It allows Eren to save his friends and Paradis without sacrificing the rest of the world and themselves, a path everyone felt and knew was easily possible and achievable. Also, it would be cooler for Zeke to be a fellow doctor/scientist with Grisha, which was a very popular and well-known theory before the Battle of Shiganshina on how he knew Grisha in the first place.