Well, for a Bleach Attack on Titan crossover fanfiction, you could have the Quincy from Bleach be interested in the Titans as a new source of spiritual energy. They might try to harness it, which could lead to a conflict with both the Soul Reapers and the people of Attack on Titan. Also, the Arrancars could be sent into the Attack on Titan world by Aizen to cause chaos and the characters from Attack on Titan would have to team up with the Soul Reapers to stop them.
In a Bleach Attack on Titan crossover, imagine the Gotei 13 sending a small group of their best Soul Reapers to investigate the Titans. They discover that the Titans are actually the result of some dark, forbidden Soul Reaper experiment gone wrong in the past. The Soul Reapers then have to decide whether to help the people of Attack on Titan by destroying the Titans or try to find a way to reverse the experiment. Meanwhile, characters like Levi could befriend some of the Soul Reapers and learn about their powers, and perhaps use some of their techniques in his own combat against the Titans.
One idea could be that the Soul Reapers have to enter the world of Attack on Titan to stop Hollows that are being created by the Titans' negative energy. Another could be that Eren Jaeger somehow gains Soul Reaper powers and uses them to fight both Titans and Hollows. Maybe Ichigo could end up in the walls and teach the Survey Corps his sword - fighting techniques to better combat the Titans.
In an 'Attack on Titan' and 'Titanfall' crossover fanfiction, there could be a plot where the world of 'Titanfall' is somehow connected to the world of 'Attack on Titan' through a rift in space - time. The Pilots could be sent to the 'Attack on Titan' world to help in the fight against the Titans. They could introduce their Titan - mech technology to the people there. The story could also focus on the cultural shock that both sides experience. The Pilots would be shocked by the primitive living conditions in 'Attack on Titan' while the 'Attack on Titan' characters would be amazed by the high - tech machinery of the Pilots.
One idea could be that Mel, a character from another universe perhaps, has some unique ability that can help the Survey Corps in 'Attack on Titan' deal with the Titans. For example, if Mel can manipulate time, she could freeze the Titans in place, giving the soldiers a chance to attack without risk. Another idea is that Mel accidentally ends up in the world of 'Attack on Titan' and has to learn the ways of survival there. She could form an unlikely friendship with Eren or Mikasa and together they discover new ways to fight the Titans.
The combination of different power systems. In Bleach, you have spiritual powers like zanpakuto and kido, while in Attack on Titan, there are the unique abilities related to the Titans. Seeing how these interact in a crossover is really cool.
Ichigo and Eren would be a very interesting pair. Ichigo has his powerful Soul Reaper and Hollow powers, and Eren has the power of the Titans. They could learn from each other, like Ichigo teaching Eren about controlling his inner power better, and Eren showing Ichigo the determination of the people in the Attack on Titan world.
A great 'bleach crossover fanfiction' idea is to cross it with One Piece. The Straw Hat crew could end up in the Soul Society. Luffy's carefree attitude would contrast with the more solemn Shinigami. Also, a Bleach and Fullmetal Alchemist crossover where the alchemic principles are applied in the Bleach universe would be really interesting.
Perhaps the friendly titan is a protector of a certain area outside the walls that the humans have not explored much. The story could be about the Survey Corps making an alliance with this titan. The titan guards the area from other dangerous titans, and in return, the humans help the titan when it gets injured. This could lead to a whole new understanding of the titan world and how they can co - exist with the friendly ones.
One idea could be to have Goku from DBZ end up in the Bleach universe and learn about soul reapers. He might mistake them for a new type of fighter and try to spar with them. Another could be a fusion of the power systems, like Ichigo getting the ability to go Super Saiyan on top of his existing powers.
One idea could be that Sasuke somehow ends up in the Bleach universe and teams up with Ichigo. They could face off against a common enemy that has powers related to both the Sharingan and the Zanpakuto. For example, a villain who can manipulate souls like in Bleach but also has the ability to copy techniques like the Sharingan.
One idea could be that the Pokemon trainers gain Shinigami powers. For example, Ash could discover he has the ability to purify Hollows in the Pokemon world, which are like dark or corrupted Pokemon. His Pikachu could also have a unique transformation related to this new power, like having a Zanpakuto - like form.
One idea could be a scenario where Ichigo and Naruto meet in a strange dimension. They initially misunderstand each other but then realize they have a common goal of protecting their respective worlds. They could team up to fight a new threat that somehow has the power to destroy both the Soul Society and the Hidden Leaf Village.