I think its weird that Eren says Zeke`s euthanasia plan is messed up and then comes up with a genocide plan killing the entire world outside paradis island. I think its his plan, but not his complete plan like it seems at the moment. This is because i dont see Isayama making the lelouch ending. At least thats what i am hoping for, because he can do better ;). It`s a combined theory of so many theories i heard or read on youtube so far.
So here`s my Ending Scenario:
Eren will use the rumbling to kill all the armies of marley and other nations who will attack paradise island after Eren activated the Rumbling. Eren is unstoppable with his army of collosal titans and kills a huge amount of arriving soldiers, tanks, planes, ships... and then wants to leave paradis island to crush the entire world. The only people who can stop Eren at this point are his friends, and he already knew that from future memories which are not shown yet, that they will come and face him. I have no idea if the survey chorps including Levi and Hangie will be that last resistance, or if they team up with Rainer, Peke, Annie,...
So in the final battle, it will only be Mikasa and Armin who stop Eren. It might be Armin, because it was so often teased that he is humanitys saviour. It might be Mikasa, to have the bitter sweet ending, because she is or was in love with Eren. So after Eren`s two childhood friends defeat him, he will have one last moment with mikasa and armin. While dying he explains them why he was such a jerk to them (in chapter 112) and why he had to kill so many innocent (because of the future memories). Maybe Eren confesses to Mikasa, one final kiss for Eremika.
The survivors of the final battle are Armin, Mikasa, Jean, Connie, Levi, Hangie and Historia. On the marleyian side it might be Falco, Gabi, Annie, Rainer,and Peke. I dont think Zeke will be alive in the End, maybe he sacrifices himself for Peke or gets killed by Levi finally.
Eren died as the villain (in the eyes of the world), but as a Hero too (regarding Eldians). After eren died, he meets ymir in the koordinate and makes her free the eldians, so no eldian will be able to transform into a titan and turns all remaining collosal titan from the rumbling back into humans.
The Titan curse is broken, there are no more Titanshifters. With the curse broken, the intermediate world of the koordinate start to crumble and the giant tree of the coordinate is collapsing. Eren will hug Ymir in the end and say "you`re free". Surprise, its not historia`s baby, ymir is the one on the "last manga panel". Isayama trolled his readers.
The last chapter might take place some years after the Rumbling and Erens Death. The Eldians, The Marleyans and the other Nations made a piece contract together. Historia signs the contract as the Representants of the Eldians and Mikasa signs as the leader of the asian clan. In the government hall we also see Armin (the hero "Helos, who defeated the devil) carrying Historia`s babygirl "Ymir", Levi and Hangie holding hands, and the rest of our beloved characters.
What do you think about the ending, too happy for the dark SnK Manga?