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.
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 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.
One interesting plot twist could be a character from 'Supernatural' getting the kagune power from 'Tokyo Ghoul'. For example, Dean suddenly having a rinkaku kagune and having to learn how to control it while still hunting demons. It would add a whole new layer to his character as he struggles with this new, inhuman power.
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.
In some crossovers, like the one with 'Bleach', a really creative plot twist is when Kaneki's kagune somehow gets linked to a Hollow's power. It completely changes the power dynamics and makes for some intense battles. The idea that his ghoul nature could be combined with Hollow abilities in such a way was unexpected and exciting.
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.
One exciting plot twist could be Ben using his alien forms in unexpected ways within the Attack on Titan world. For example, using Heatblast to melt through the Titans' hard exteriors in a crucial battle. Another could be a character from Attack on Titan getting the Omnitrix and having to learn how to use it. This would completely change the power dynamics in the story.
Perhaps a plot twist where the Mist from Percy Jackson's world starts to affect Tokyo Ghoul. The Mist could make humans in Tokyo see ghouls as something completely different, like mythical creatures from their own Japanese folklore. This would not only change how the humans in Tokyo Ghoul react to ghouls, but also how the ghouls themselves perceive their place in the world and their relationship with humans.
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.
The character interactions can be really fascinating. Imagine Mikasa Ackerman teaming up with a powerful Pokemon like Charizard. She could ride on its back and use her combat skills along with Charizard's fire - breathing abilities to take on Titans. Also, the idea of the Pokemon having to adapt to the world of Attack on Titan, where there are different rules and a lot more danger, is an aspect that can be explored in many interesting ways.