Well, perhaps in this 'immortal ichigo twilight crossover fanfiction', Ichigo arrives in the Twilight world during a time of great unrest. The Volturi are on the verge of a major power play. Ichigo's immortality gives him an edge in this dangerous new world. He teams up with Bella and Edward. Ichigo's combat skills and reiatsu powers could be a huge asset against the Volturi's army. They could form an unlikely alliance to protect Forks and the rest of the world from the Volturi's tyranny.
In this fanfiction, it could be that Ichigo's immortality is a curse of sorts. He's pulled into the Twilight realm where his spiritual energy attracts all kinds of supernatural attention. The werewolves sense a strange new power in him and are initially hostile. The vampires are intrigued. He has to navigate this world where the rules are so different from his own. He might discover that his immortality has a connection to an ancient vampire or werewolf legend in the Twilight world, and this discovery could change the balance of power in the ongoing battles between the different factions.
One possible interesting plot could be Ichigo finding himself in a world where he has no identity or recognition. He could start a journey to regain his status while facing challenges from the new world's rules and characters. Maybe he has to form alliances with unlikely characters to survive and prove his worth.
Ichigo and Edward would be really interesting. Ichigo, with his brash and head - on attitude, would contrast with Edward's more brooding and cautious nature. They could have debates about what it means to be immortal, as Ichigo has his own reasons for being immortal that are different from Edward's vampiric immortality. Also, Ichigo's combat skills could be something that Edward admires and wants to learn from, while Ichigo might be interested in Edward's mind - reading ability.
One interesting plot could be that Ichigo and Naruto are brought together in a strange alternate dimension. In this dimension, they have to team up to fight a new common enemy that has powers combining the Hollow and the Nine - Tails. They start off not understanding each other's abilities but gradually learn and combine their powers in unique ways to defeat the enemy.
One possible plot could be that Ichigo somehow gets transported to the world of High School DxD. He might initially be seen as a threat by the devils and angels there due to his unknown powers. But as they learn about his motives of protecting people, they start to work with him. Ichigo could also teach the characters in High School DxD about his unique fighting style and the concept of Hollow and Soul Reaper powers, leading to new and exciting battles against common enemies.
One possible plot could be that Wolverine somehow ends up in the Twilight universe. Maybe he's on a mission that goes wrong and he gets transported there. He then meets the vampires and werewolves in Twilight. The vampires might be intrigued by his healing factor and his adamantium claws. And the werewolves could see him as either a threat or an ally. He could get caught in the middle of the conflict between the Cullens and the Volturi, using his combat skills to tip the balance.
One possible plot could be that the vampires from Twilight somehow end up in the world of Glee. Maybe they are drawn there because of a unique musical energy. The Cullens could enroll in McKinley High and join the Glee club. Edward, with his musical talent, could be a star singer, but his vampire nature would cause some interesting conflicts, like how to handle his super strength during dance numbers or when handling musical instruments without breaking them.
One interesting concept could be Ichigo's power evolution in a crossover setting. Since he's immortal, he might have the time to fully master and combine his various powers in unique ways. For example, his Shinigami, Hollow, and Quincy powers could blend more seamlessly, creating new and unforeseen abilities.
One idea could be that Naruto, with his newfound immortality, somehow ends up in the Twilight universe. He might be mistaken for a new kind of vampire due to his enhanced abilities. Another plot could involve the Twilight characters learning ninja techniques from Naruto. For example, Bella could learn how to better protect herself using Naruto's shadow clone jutsu.
Perhaps Bella could be a key figure in a crossover. She could be a 'dragon rider' in a sense, using her connection with Edward (similar to the bond between a dragon and its rider in Game of Thrones). Her immunity to vampire powers could translate to some sort of immunity to the dark magic in Westeros. The werewolves from Twilight could be allies with the Stark direwolves, and together they fight against common enemies, like an army of the dead led by a Volturi - like figure.
One possible plot could be that the Volturi from Twilight discover the existence of demigods from the Percy Jackson world. They see the demigods as a threat to their vampire supremacy. Percy and his friends have to team up with some of the more friendly vampires, like the Cullens, to fend off the Volturi. This would bring together the magic and powers of both universes in an epic battle.