Perhaps Eren and his friends from Attack on Titan enter the Unordinary universe and are mistaken for having abilities. They then have to navigate the complex social hierarchy of the Unordinary world while also trying to find a way back to their own world. And the Titans might start showing up in the Unordinary world, causing chaos.
In this fanfiction, the characters from both worlds could team up. The titan shifters might share their powers with some of the Unordinary characters, and together they could fight against a common enemy. It could be that a new type of titan - like creature is created in the Unordinary world due to some sort of experiment gone wrong, and both groups have to put aside their differences to stop it.
One possible plot could be that the Titans from Attack on Titan are somehow connected to the alien threats in Destiny. The Guardians could be sent to the world of Attack on Titan to investigate the sudden appearance of these giant creatures. They might initially struggle to understand the unique nature of the Titans, which are different from the enemies they usually face. Along the way, they could team up with the Survey Corps, sharing their advanced technology and combat skills. This would lead to some epic battles where the combined forces of Guardians and the Survey Corps fight against the Titans, while also trying to figure out the mystery behind their existence.
Well, perhaps the titans could be like the animatronics in FNAF. The Survey Corps could be trapped in a titan - infested area that has some strange FNAF - like mechanics. For example, the titans are only active at night and there are 'rules' to avoid them similar to the rules in FNAF games.
John from Unordinary could also be a great focus. If he meets the characters from Attack on Titan, he might be both intrigued and a bit scared. His ability to copy powers could be really useful against the Titans, and his own internal struggle with power could be contrasted with the characters from Attack on Titan who have their own power - related issues.
A great plot for such a fanfic could be that Mikasa's Titan form is a result of a connection to an ancient Titan power. As she becomes a Titan, she starts to have visions of the past, of how the Titans came to be. She then decides to use this knowledge and her new form to try and end the Titan - human conflict once and for all. Along the way, she has to face her friends who are initially scared of her but eventually come to accept her new form as she proves that she is still the same Mikasa deep down, just in a different body.
One idea could be that the Overwatch heroes are transported to the Attack on Titan world. Tracer could use her time - traveling abilities to outmaneuver the Titans. Maybe Reinhardt's shield would be crucial in protecting the Survey Corps from Titan attacks.
Well, they might have a deep conversation about their different views on the world. Eren could try to convince Zeke of his more radical approach to saving the Eldians, while Zeke might push back with his own ideas of euthanasia for the Eldians. It could be a very tense encounter.
Well, perhaps she is reborn as a member of a noble family in the Attack on Titan world. This gives her access to resources and influence. She could use this new - found power to reform the military or start research into alternative ways to deal with the Titans. She might also get involved in the complex political web within the walls, trying to bring about peace and unity among the different factions.
One possible plot could be that Naruto and his friends somehow end up in the world of Attack on Titan. Naruto's ninja skills would be really useful against the Titans. For example, his shadow clone jutsu could be used to distract the Titans while the Survey Corps tries to attack their weak points. Also, Sasuke's sharingan might be able to see some hidden details about the Titans that the people in the Attack on Titan world don't know. It would be an epic adventure as they combine their powers to fight the common enemy - the Titans.
Well, it could be about Sephiroth using his incredible combat skills from Final Fantasy in the world of Attack on Titan. Maybe he teams up with the Survey Corps to take on the Titans. He might use his Masamune sword to slice through the Titans' nape with ease, bringing a new level of badassery to the fight against the Titans.
One challenge is combining the two very different universes. Overwatch has a more futuristic, technology - based setting, while Attack on Titan is more medieval - ish with its walled cities and limited technology. So, making the two settings blend together in a believable way can be tough.