One unique feature is the combination of different power systems. You have the spiritual powers from Bleach like Ichigo's Zanpakuto and Rukia's Soul Reaper abilities, and then you have the diverse powers of Pokemon. For example, a Pokemon's electric attack might interact in an unexpected way with Ichigo's Getsuga Tensho.
Well, one popular ichigo rukia pokemon crossover fanfic might be 'A Soul Reaper's Pokemon Journey'. In this fic, Ichigo and Rukia find themselves in the Pokemon world and have to adapt to catching Pokemon while still maintaining their Soul Reaper powers. It's a really interesting take on combining the two universes.
One idea could be that Aizen kidnaps them to use Ichigo's power and Rukia's knowledge of Soul Society for his own dark plans. He might want to study Ichigo's unique Hollow - Shinigami powers up close and use Rukia as a bargaining chip with Soul Society.
Yes, there might be some fanfics out there with such a plot. Fanfiction often explores different scenarios and relationships. However, it's important to note that these are fan - created works and not part of the official canon.
Aizen might be motivated by power. Ichigo has a very unique power set that Aizen may want to harness for himself. And Rukia, being from Soul Society, could be used as a way to gain information or leverage against Soul Society.
I've read a few fanfics with these characters. A really interesting one was where Byakuya had to team up with Ichigo to rescue Rukia from a new and powerful enemy. The fanfic did a great job of showing Byakuya's reserved yet powerful nature, Ichigo's determination, and Rukia's strong will. It was full of action scenes and also some really touching moments between the characters. You can often find such fanfics on dedicated fanfic websites by using the relevant tags.
Another reason is that the world they exist in, the Soul Society and the world of Hollows, is a rich and interesting backdrop. Fanfic writers can use this world as a canvas to paint different stories for Ichigo and Rukia. It allows for a lot of creativity in terms of plot, character growth, and relationship building.
One common theme could be friendship. Given the relationships between Ichigo, Rukia, and Hiyori in the original Bleach series, fanfics might explore how their friendships develop further. Another theme could be adventure, as they often face various challenges in the series.
Maybe Aizen has created a false asylum as part of a new experiment. He lures Ichigo and Rukia into it, and they have to find a way to escape while also dealing with the illusions and traps Aizen has set up. In the process, they could learn more about Aizen's motives and past, and perhaps even find a way to defeat him once and for all.
A good Bleach Ichigo x Rukia fanfic often has well - developed characters. The author should stay true to their personalities as shown in the original series. Also, a compelling plot is key. It could be a story about them facing a new threat together or exploring their feelings in a unique situation.