Well, in this crossover, the Survey Corps members like Armin would be fascinated by the intelligence of some ghouls. They might try to communicate with them to gain knowledge about their powers. Ghouls like Touka would be in a difficult situation. She would be in a world full of titans, which are much larger and more difficult to deal with than the CCG in her own world. She might have to learn new fighting techniques from the Survey Corps. And characters like Erwin would see the ghouls as a valuable asset, trying to recruit them to fight the titans.
The characters would interact in very interesting ways. For instance, Levi from Attack on Titan might be initially suspicious of the ghouls. But once he sees their combat abilities, he could try to recruit them or at least work with them. Kaneki, on the other hand, might be in awe of the Survey Corps' determination. He could learn a lot from their teamwork and discipline. And the ghouls' concept of a 'family' within Anteiku could be something that the soldiers of the Survey Corps might find strange but also endearing.
The Survey Corps' ODM gear could be used in new and creative ways in a crossover. They could use it to quickly move around the Tokyo Ghoul cityscape, giving them an advantage against the ghouls. Meanwhile, the ghouls' kagune could be used to attack Titans in ways the Attack on Titan characters haven't seen before. For example, a ghoul could use its kagune to slice through a Titan's nape from a distance.
A crossover story could be really exciting. In this kind of fanfiction, we might see the Survey Corps from Attack on Titan having to deal with the ghouls from Tokyo Ghoul. Maybe Eren and his friends would be confused at first about these new, strange creatures that are part human, part something else. They could initially mistake the ghouls for Titans. And the ghouls, on the other hand, would be equally shocked by the Titans and the advanced technology the Attack on Titan characters use. It would be a clash of two very different but equally dark and violent worlds.
One interesting plot twist could be the ghouls in the Attack on Titan world. Imagine if ghouls were hidden among the titans. They could feed on the bodies of dead titans or even on the soldiers without being detected at first. Another could be the Ackermans having some sort of connection to the ghoul powers. Maybe their enhanced strength is related to a dormant ghoul gene. And a third could be the Scouts finding a way to use ghouls' kagune to fight the titans more effectively, like creating weapons out of kagune material.
The characters would interact in very unique ways. For example, Harry Potter might be initially cautious around the ghouls from Tokyo Ghoul. He's used to dealing with Voldemort and his Death Eaters, but ghouls are a completely different type of threat. Ken Kaneki, being a half - ghoul, could teach the wizards about the nature of ghouls, and in return, the wizards could show him some amazing magic spells.
The characters could interact through a shared goal. For example, if there's a new type of Titan threat that neither the 'Attack on Titan' characters nor Mel can handle alone, they have to work together. Eren might initially be distrustful of Mel, but as they face challenges together, they start to trust each other. Mikasa could be protective of Mel at first, seeing her as a vulnerable outsider in their dangerous world.
The 'Attack on Titan' characters would be quite shocked by the technology and the general chaos in the 'Gintama' world. Characters like Armin would be very interested in all the strange inventions and the way society functions in 'Gintama'. He might try to understand the economic and political systems there. And the 'Gintama' characters would be in awe of the 'Attack on Titan' characters' ability to fight Titans. They could team up to face common enemies or just have a series of comical misunderstandings that lead to all sorts of adventures.
A cool plot twist could be a ghoul - Titan hybrid. Imagine a Titan with the regenerative abilities of a ghoul. This would pose a whole new level of threat to the characters from both universes. Also, the idea of the ghouls' coffee being a valuable resource in the Attack on Titan world could be an interesting twist. Since coffee is so important to ghouls, it could have some unforeseen effects on the Titans or the people in that world.
The Jedi would likely approach the ghouls with caution at first. They would try to understand their nature and see if there's a way to coexist. For example, Obi - Wan Kenobi might try to negotiate with the ghoul leaders. The Sith, on the other hand, might see the ghouls as potential minions or tools. Darth Maul could try to use the chaos of the Tokyo Ghoul world to his advantage.
Luffy would probably be really excited to meet the Survey Corps. He'd see them as cool warriors and want to team up right away. He might even try to convince them to join his crew. Eren, on the other hand, would be a bit more suspicious. He's used to a world full of danger and betrayal, so he might not trust the Straw Hat crew immediately. But as he sees their loyalty to each other, he could start to warm up to them.