I do not own Shingeki no Kyojin
Eren suddenly woke up when he realized he was standing shoulder to shoulder with nine others. His last memory was Mikasa beheading him and then kissing the severed head. That was sweet and creepy all at the same time.
So he had come back in time for the graduation of 104th batch. He started thinking of how everything would go this time from Trost to Hannes but then stopped. Was he retarded? Why would he want to repeat everything?
What he still had were his hand to hand combat skills learned from Annie and polished further. What he lacked was the hardening ability. Turned out it wasn't just knowledge but actual physical thing he got from that bottle in the Reiss cave.
He sneaked out at night after the graduation party and stole the extra Ice-burst stone not being used by the 3DMG. It wasn't much but still more than enough. He did not have technical expertise of Hanji to make thunder spears. Or even pressure based mines for that matter but he didn't need to.
The next day he was on top of the wall. The shenanigans of Connie, Thomas, Mina and Sasha were the same. Then colossus titan appeared staring menacingly. Everyone was blown off the wall except Eren who just lied down. He idly wondered if Samuel was crippled again. The next moment, another huge explosion happened and the colossus stumbled to one side before falling on the ground creating another huge blast.
Eren had simply placed the Ice-burst stone buried slightly under the soil where colossus titan's left leg will appear based on where he did last time. It needed no engineering as the heat of the colossus set off the ignition. Bertholt's left leg was blown up and he was too heavy to balance on one leg. He fell down to gravity banging his head in the process as his arm was too long and skinny to slow the fall.
While he was still stunned, Eren swooped in, cut him out without his limbs. Then he transformed and ate Bertholt without any hesitation. Next time he transformed, he would be colossus size. That wasn't all good as it had bad stamina efficiency and with the slow speed, his fighting skills will be useless. But he needed it to deal with Reiner before he got the armor. He then ran straight south to deal with Annie.
Annie was sprinting straight north with a horde of titans behind her when she saw a rogue titan coming from ahead. She pulled her arm back for a punch expecting a win though she had felt it was dangerous. The titan dodged and hit her with an open palm in the chest instead of a punch. She flew back directly towards the horde, her mind stunned. The horde was soon consuming her. She tried to fight back but they were too many.
In the end she got eaten. That titan de-transformed but got eaten by another and that continued. Eren was curious about what would happen when a male titan ate the female titan but steam obscured his vision and he didn't dare to get close. This was a nice and simple way of getting rid of titans in locality. What was left in the end was a single naked female. He picked her up and ran back towards wall Rose but away from Trost. Steam must have pushed his friends off the wall and also obscured their vision so making up a story later should not be hard.
After hiding the unknown woman he went back to his friends who were in half grieving and half hopeful mode. Titans still trickled in Trost but much slower than last time and so were easy to deal with. Reiner was panicking as his comrades hadn't returned when Eren sneak-attacked him. Unfortunately even cutting into his neck wasn't successful as Reiner had learned to transfer his consciousness.
They both transformed, Reiner shocked by the colossus titan so Eren stomped him in the ground and kept doing it to shatter the armor. Then he ripped Reiner out and ate him. He now also had partial armor ability as his body was too big for full armor. Then he used steam to disappear and rejoin his friends.
When the Survey corps came back, everything was resolved. Eren was still wondering when and how to tell Mikasa and Armin about everything and if they will become his enemy again. A part of him resented their self-righteousness when they did not even have any other solution. Its not like he enjoyed mass-murder, there was just no other way. Next he started planning for the arrival of Zeke and Pieck.
This fandom is filled with travel back to the past and re-do stories and I have read so many of them. Its also a common theme in Harry Potter and One Piece fandom. It makes sense in Harry Potter but One Piece is a story about adventure and living without regrets so it doesn't make sense.
Not a single one of these Shingeki stories dared to change anything, simply using canon events as landmarks. The limit of change is saving Petra and everyone is happy in the comments. And even that character does not change anything. Even adding SI and OC's don't change anything making them zero-sum characters and completely pointless.
This story is too compressed as I don't have patience to flush out all the details. Its not good but its still better than the canon rehash. I even put additional rule that Eren cannot borrow help from Armin and Mikasa or anyone else to control the butterfly effect. That he must do it all by himself and yet this was the result of natural flow. Is it really that difficult to let your imagination run a little? I hope someone will write a better version of this someday. I also plan to do a reverse of this where Reiner gets future memories.