Another theme is the exploration of power. Dark Ichigo has a significant amount of power, and in crossover fanfictions, it's interesting to see how that power measures up against the powers in other fictional worlds. For example, if he crosses into a world with magic, how does his reiatsu compare to the magic system there? It's all about power dynamics.
One potential antagonist could be an ancient evil that is awakened by Ichigo's newfound power. This entity might see Ichigo as a threat or as a means to achieve its own dark goals.
One popular 'dark ichigo crossover fanfiction' is 'Dark Ichigo in the World of Naruto'. In this story, Dark Ichigo enters the Naruto universe and causes all kinds of chaos with his powerful and dark abilities. His interactions with Naruto and other characters are really interesting as he challenges their beliefs and power systems.
One interesting concept could be the exploration of Ichigo's new powers in a 'dark godlike' form. It might involve him having enhanced combat abilities that are both awe - inspiring and a bit terrifying.
One interesting character to crossover with Ichigo is Light Yagami from Death Note. The clash between Ichigo, who is a protector, and Light, who is a morally ambiguous character with the power to kill, would be very engaging. Saitama from One - Punch Man could also be a great choice. Saitama's overpowered nature and Ichigo's journey to become stronger would make for an interesting story. And what about Alucard from Hellsing? His vampire powers and Ichigo's powers related to the afterlife would create a really dark and exciting crossover.
Aizen Sousuke would be an interesting crossover character. Since he's so powerful and manipulative in the Bleach universe, seeing him in a different setting with Ichigo could create some really complex and intense storylines. They could be forced to work together against an even greater evil in a crossover universe.
Ichigo could meet Frodo Baggins. Frodo, being from a very different kind of world, would find Ichigo's situation strange. But they could bond over their shared sense of being out of place in some ways. Frodo could teach Ichigo about the importance of perseverance in the face of great odds, and Ichigo could protect Frodo from any potential threats in this crossover world with his swordsmanship.
One idea could be that Ichigo and Naruto are brought together by some sort of interdimensional rift. They initially don't understand each other's powers but soon realize they share a common goal of protecting their friends and worlds. They could team up to fight a powerful enemy that threatens both their universes.
Obviously, Ichigo and Dark Ichigo are the central characters. Ichigo, with his complex personality and powers, is always a key figure. Dark Ichigo, being his inner Hollow, brings a lot of drama and conflict. The OC also becomes popular depending on how well - written they are. If the OC has unique powers or a really interesting backstory, they can gain a following among fans of the fanfiction.
One common theme is the battle between good and evil within Ichigo. His normal self represents the good, while Dark Ichigo represents the more primal and sometimes malevolent side. Another theme is self - discovery. As Ichigo grapples with Dark Ichigo, he often discovers new aspects of his own power and personality.